M4300 - MACs age out instantly with STP enabled (tcnguard is enabled)

We appear to have hit an issue whereby MAC addresses associated with an interface configured in dot1q-tunnel (double VLAN) mode, whereby MACs age out instantly and therefor results in unicast traffic being broadcast to all VLAN member ports.

We originally observed this behaviour in 12.0.11.15 and can confirm that the problem also occurs after having upgraded the stack of 2 x M4300-24X24F and 2 x M4300-96X to 12.0.13.7.

The problem occurs the moment we enable spanning tree on the interface. I'm under the impression that 'guard root' and 'tcnguard' should remediate against these issues but presume a bug as a TCN, even if it bypasses the TCN guard filter, shouldn't result in MAC addresses aging out immediately.

Herewith the interface configuration and MAC table when things are working as expected:

(M4300-24X24F) #show running-config interface 2/0/21

!Current Configuration:
!
interface  2/0/21
description 'dot1q-tunnel vlan 11'
spanning-tree guard root
spanning-tree tcnguard
no spanning-tree port mode
mtu 12270
vlan pvid 11
vlan ingressfilter
vlan participation auto 1
vlan participation include 11
exit

(M4300-24X24F) #show mac-addr-table vlan 11

Address Entries Currently in Use............... 5483

   MAC Address            Interface            Status
-----------------  -----------------------  ------------
00:C0:89:21:E9:20  2/0/21                   Learned
00:C0:89:3D:52:8C  2/0/21                   Learned
02:BD:36:44:92:8D  lag 22                   Learned
02:CE:14:AD:F7:A0  lag 22                   Learned
08:55:31:30:21:C8  2/0/21                   Learned
08:55:31:C1:1D:FF  2/0/21                   Learned
4C:5E:0C:39:03:24  2/0/21                   Learned
4C:5E:0C:E1:52:E4  lag 22                   Learned
74:4D:28:05:76:C0  2/0/21                   Learned
74:4D:28:07:A9:62  2/0/21                   Learned
74:4D:28:07:B1:28  2/0/21                   Learned
C4:AD:34:6A:49:75  lag 22                   Learned
C4:AD:34:71:A6:8F  2/0/21                   Learned
C4:AD:34:85:35:2E  2/0/21                   Learned
C4:AD:34:85:35:30  2/0/21                   Learned
C4:AD:34:BD:62:9E  2/0/21                   Learned
CC:2D:E0:BD:4F:FC  lag 28                   Learned
CC:2D:E0:E4:B3:86  lag 26                   Learned
D4:CA:6D:01:CB:BF  2/0/21                   Learned
E4:8D:8C:3A:E0:A4  lag 22                   Learned

The mac address table remains in this state, without churn, until we enable STP on the interface. In the following examples we ran the 'show mac-addr-table vlan 11' command in quick succession (up arrow key and then enter) with 1 second or less time between each command. The system log shows no event being logged when this behaviour exhibits itself:

(M4300-24X24F) #configure

(M4300-24X24F) (Config)#interface 2/0/21
(M4300-24X24F) (Interface 2/0/21)#spanning-tree port mode
(M4300-24X24F) (Interface 2/0/21)#end
(M4300-24X24F) #show mac-addr-table vlan 11

Address Entries Currently in Use............... 5474

   MAC Address            Interface            Status
-----------------  -----------------------  ------------
02:BD:36:44:92:8D  lag 22                   Learned
02:CE:14:AD:F7:A0  lag 22                   Learned
08:55:31:30:21:C8  2/0/21                   Learned
4C:5E:0C:E1:52:E4  lag 22                   Learned
C4:AD:34:6A:49:75  lag 22                   Learned
CC:2D:E0:BD:4F:FC  lag 28                   Learned
CC:2D:E0:E4:B3:86  lag 26                   Learned
E4:8D:8C:3A:E0:A4  lag 22                   Learned

(M4300-24X24F) #show mac-addr-table vlan 11

Address Entries Currently in Use............... 5473

   MAC Address            Interface            Status
-----------------  -----------------------  ------------
02:BD:36:44:92:8D  lag 22                   Learned
02:CE:14:AD:F7:A0  lag 22                   Learned
4C:5E:0C:E1:52:E4  lag 22                   Learned
C4:AD:34:6A:49:75  lag 22                   Learned
CC:2D:E0:BD:4F:FC  lag 28                   Learned
CC:2D:E0:E4:B3:86  lag 26                   Learned
E4:8D:8C:3A:E0:A4  lag 22                   Learned

(M4300-24X24F) #show mac-addr-table vlan 11

Address Entries Currently in Use............... 5476

   MAC Address            Interface            Status
-----------------  -----------------------  ------------
02:BD:36:44:92:8D  lag 22                   Learned
02:CE:14:AD:F7:A0  lag 22                   Learned
4C:5E:0C:39:03:24  2/0/21                   Learned
4C:5E:0C:E1:52:E4  lag 22                   Learned
74:4D:28:07:A9:62  2/0/21                   Learned
C4:AD:34:6A:49:75  lag 22                   Learned
CC:2D:E0:BD:4F:FC  lag 28                   Learned
CC:2D:E0:E4:B3:86  lag 26                   Learned
D4:CA:6D:01:CB:BF  2/0/21                   Learned
E4:8D:8C:3A:E0:A4  lag 22                   Learned

(M4300-24X24F) #show logging buffered

Buffered (In-Memory) Logging        : enabled
Buffered Logging Wrapping Behavior  : On
Buffered Log Count                  : 3071
Buffered Log Threshold (lines)      : 160

<13> Jun 30 22:34:55 M4300-24X24F-1 CMDLOGGER[emWeb]: cmd_logger_api.c(83) 3000695 %% CLI:154.119.67.52:davidh:show mac-addr-table vlan 11
<13> Jun 30 22:34:54 M4300-24X24F-1 CMDLOGGER[emWeb]: cmd_logger_api.c(83) 3000691 %% CLI:154.119.67.52:davidh:show mac-addr-table vlan 11
<13> Jun 30 22:34:54 M4300-24X24F-1 CMDLOGGER[emWeb]: cmd_logger_api.c(83) 3000690 %% CLI:154.119.67.52:davidh:show mac-addr-table vlan 11
<13> Jun 30 22:34:50 M4300-24X24F-1 TRAPMGR[dot1s_task]: traputil.c(795) 3000676 %% Spanning Tree Topology Change Initiated: 0, Interface: 2/0/21
<13> Jun 30 22:34:50 M4300-24X24F-1 TRAPMGR[dot1s_task]: traputil.c(795) 3000675 %% Spanning Tree Topology Change: 0, Unit: 1
<13> Jun 30 22:34:49 M4300-24X24F-1 CMDLOGGER[emWeb]: cmd_logger_api.c(83) 3000671 %% CLI:154.119.67.52:davidh:spanning-tree port mode
<13> Jun 30 22:34:36 M4300-24X24F-1 CMDLOGGER[emWeb]: cmd_logger_api.c(83) 3000631 %% CLI:154.119.67.52:davidh:interface 2/0/21
<13> Jun 30 22:34:32 M4300-24X24F-1 CMDLOGGER[emWeb]: cmd_logger_api.c(83) 3000614 %% CLI:154.119.67.52:davidh:configure