SXR50 SNMP traffic intermittently sent to wrong VLAN ?

[Animal7857]

SXR50 / SXS50 with FW 4.3.0.140 in AP mode

Wireless 1:  Vlan unchecked, default(1).  ( Management VLAN )
Wireless 2:  Vlan 10    ( restricted / IOT devices )
Wireless 3:  Vlan 50    ( client PC's main LAN )

Router is connected to SXR50 WAN port configured as default / Trunk mode.

I have  a strange intermittent issue with clients on vlan 50 sending SNMPv1 requests ( UDP port 161 ) to a wifi printer on VLAN 10.  For several hours a day the requests are forwarded back to the router on VLAN 50 instead of to the printer.  At other times it works fine.  Also works fine for all other traffic e.g. ICMP and TCP/HTTP.

Neither device is on vlan 1 aka br-lan but due to timing of the interface initialization the SXR50 does select vlan 50 as it's default route.

The normal flow is:

PC  -> SXS50 Vlan 50 -> SXR50 -> Router -> SXR50 VLAN 10 -> SXS50 -> Printer

So from Router perspective when SNMP is working it looks just like this HTTP example:

04:48:23.825209 <PC MAC>(oui Unknown) > <ROUTER MAC>(oui Unknown), ethertype 802.1Q (0x8100), length 64: vlan 50, p 0, ethertype IPv4, (tos 0x0, ttl 128, id 27620, offset 0, flags [DF], proto TCP (6), length 40)
192.168.2.6.53562 > Brother-J480DW.lan.80: Flags [.], cksum 0xd49c (correct), seq 1106, ack 175, win 1025, length 0
04:48:23.825235 <ROUTER MAC>(oui Unknown) > <PRINTER MAC> (oui Unknown), ethertype 802.1Q (0x8100), length 58: vlan 10, p 0, ethertype IPv4, (tos 0x0, ttl 127, id 27620, offset 0, flags [DF], proto TCP (6), length 40)
192.168.2.6.53562 > Brother-J480DW.lan.80: Flags [.], cksum 0xd49c (correct), seq 1106, ack 175, win 1025, length 0
04:48:23.833589 <PRINTER MAC> (oui Unknown) > <ROUTER MAC>(oui Unknown), ethertype 802.1Q (0x8100), length 1072: vlan 10, p 0, ethertype IPv4, (tos 0x0, ttl 64, id 2, offset 0, flags [DF], proto TCP (6), length 1054)
Brother-J480DW.lan.80 > 192.168.2.6.53562: Flags [P.], cksum 0xa970 (correct), seq 175:1189, ack 1106, win 7655, length 1014: HTTP
04:48:23.833605 <ROUTER MAC>(oui Unknown) > <PC MAC>(oui Unknown), ethertype 802.1Q (0x8100), length 1072: vlan 50, p 0, ethertype IPv4, (tos 0x0, ttl 63, id 2, offset 0, flags [DF], proto TCP (6), length 1054)
    Brother-J480DW.lan.80 > 192.168.2.6.53562: Flags [P.], cksum 0xa970 (correct), seq 175:1189, ack 1106, win 7655, length 1014: HTTP

However for SNMP and ONLY SNMP the requests are not bridged to the printer on vlan 10. Instead they are sent back to VLAN 50.  I have confirmed the directions for the 3 packets are IN, OUT, IN.  My router rejects the last one as an invalid input with its own source MAC.

04:27:56.916043 <PC  MAC>(oui Unknown) > <ROUTER MAC> (oui Unknown), ethertype 802.1Q (0x8100), length 124: vlan 50, p 0, ethertype IPv4, (tos 0x0, ttl 128, id 27424, offset 0, flags [none], proto UDP (17), length 106)
    192.168.2.6.65503 > Brother-J480DW.lan.161: [udp sum ok]  { SNMPv1 { GetRequest(63) R=19264  25.3.2.1.5.1 25.3.5.1.1.1 25.3.5.1.2.1 } }
04:27:56.916089 <ROUTER MAC> (oui Unknown) > <PRINTER  MAC> (oui Unknown), ethertype 802.1Q (0x8100), length 124: vlan 10, p 0, ethertype IPv4, (tos 0x0, ttl 127, id 27424, offset 0, flags [none], proto UDP (17), length 106)
    192.168.2.6.65503 > Brother-J480DW.lan.161: [udp sum ok]  { SNMPv1 { GetRequest(63) R=19264  25.3.2.1.5.1 25.3.5.1.1.1 25.3.5.1.2.1 } }
04:27:56.916211 <ROUTER MAC> (oui Unknown) > <PRINTER  MAC> (oui Unknown), ethertype 802.1Q (0x8100), length 124: vlan 50, p 0, ethertype IPv4, (tos 0x0, ttl 127, id 27424, offset 0, flags [none], proto UDP (17), length 106)
    192.168.2.6.65503 > Brother-J480DW.lan.161: [udp sum ok]  { SNMPv1 { GetRequest(63) R=19264  25.3.2.1.5.1 25.3.5.1.1.1 25.3.5.1.2.1 } }

I can't figure out how traffic is changing VLAN's without changing destination.

Tempted to just block ALL SNMP traffic but if there is some way to make this work keeping the printer status reports would be handy.