I think it is a UBNT issue - this is what I posted over there:
I just checked myself about 5 min ago - it's a special MAC address. It's almost certainly consummed by the switch, that's why it is not getting through.
The MAC address on my test network in use for Mikrotik RoMON, I believe, is:
Destination: Spanning-tree-(for-bridges)_88:bf (01:80:c2:00:88:bf)
which Wireshark flags as Spanning tree. From 802.1D-2004, table 7-10, this is outside the scope of the reserved range, which fall under the requirement:
7.12.6 Reserved addresses
Frames containing any of the group MAC Addresses specified in Table 7-10 in their
destination address field shall not be relayed by the Bridge.
The reserved range, from table 7-10, is
01-80-C2-00-00-00 to 01-80-C2-00-00-0F
I suspect that the internal filter on the UBNT switch maybe matches only 01-80-C2 or something like that, so drops the frame per 802.1D-2004, 7.12.6. Easy to test, but the vendor should be able to confirm.
Based on this, without any further documentation to the contrary, it is a UBNT bug incorrectly dropping these frames. Anyone else have any thoughts to refute the logic?