Certainly the non-standard ethertype 0x88bf could be dropped by some defaults on a cisco etc., something to look for.
You may also want make sure the Mikrotik is sending RoMON out too.... Depending on how your dealing with VLAN (e.g. via switch chip or vlan-filtering or "dumb switch"), it's possible it might not even be leaving the Mikrotik.
I say this since there are a lot of "notes" in the various Bridging/Switch help.mikrotik.com docs,
like this one, with potential "gotchas":
Bridge HW vlan-filtering was added in the RouterOS 7.1rc1 (for RTL8367) and 7.1rc5 (for MT7621) versions. The switch does not support other ether-type 0x88a8 or 0x9100 (only 0x8100 is supported) and no tag-stacking. Using these features will disable HW offload.
See also
viewtopic.php?t=181936 . Again, not sure that your issue, more highlighting that the 0x88bf ethertype is what you need to see/allow on the L2 switching plane (& ethertype 0x8100 = "normal" VLANs) .