SXK50 snmp traffic sent to wrong vlan ?

[Animal7857]

SXR50 in AP mode connected directly to router via WAN port. FW 4.3.0.140

3 VLANS:

Wireless 1  : default / 1
Wireless 2  : IOT / 10
Wireless 3  : LAN / 50

For traffic from a PC in vlan 50 to a Printer in vlan 10 the normal flow is:

( Ethernet mac src/dest, IP src/dest of course always pc/printer )

SRC: PC, DST: Router, VLAN 50  -> SXS50 -> SXR50 -> router -> SRC: Router, DST: Printer, VLAN 10 -> SXR50 -> SXS50 -> Printer

So from router I see:
Vlan 50 SNMP request in  ( pc src, router dest)
Vlan 10 SNMP request out ( router src, printer dest )
Vlan 10 SNMP reply in  ( printer src, router dest )
Vlan 50 SNMP reply out  ( router src, printer dest )

However a couple times a day for 30 minutes to a couple hours I get:

Vlan 50 SNMP request in ( pc src, router dest)
Vlan 10 SNMP request out ( router src, printer dest )
Vlan 50 SNMP request in ( router src, printer dest )

That causes router to complain about receiving packets with its own MAC and could not work anyway because the printer is not on vlan 50.

While this is happening other traffic such as http works fine.  I have only noticed it with SNMP ( UDP port 161 ) but it could be that UDP is garbled while TCP works fine since I don't have much UDP traffic between subnets.

I can't see any obvious way or reason this could happen.  Routing should change source mac and a loop would not hop vlan's.  Is it possible that the network acceleration is misclassifying the router input on vlan 50 and router output on vlan 10 as the same flow ?

Easy solution is to just block ALL SNMP traffic since I don't need it anyhow but I'm puzzled how packets end up on the wrong vlan.

Oh, my router uses the same MAC for all VLANS.