I stand corrected, not the same brand 
. I’m using a Ubiquiti AP-AC-LR. It behaves in the same way as you describe above.
Regarding my question above, I found an answer in another thread (http://forum.mikrotik.com/t/css326-24g-2s-rm-vlans/107897/1):
Since there is not an option in 2.x to specify whether to leave as-is, strip, or add VLAN headers to egressing packets, do packets always egress untagged on a port where the default VLAN ID of the port matches that of the packet, and tagged if the default VLAN ID does not match?
Yes, your assumption is correct.
Cheers
- Alex