The cause of the problem is only showing with the Air fiber X5 devices ubnt
Since Mikrotik does not know that there is a WiFi bridge or AP or whatever (it can only look until the UTP port of the Airbibre), the message is NOT related to the radio part of the Airfibre. Check the cable to the Airfibre.
I also noticed, that some LAN card manufacturers (certain Intel cards, 3COM etc…) hev problem connecting by UTP to switches. Including Mikrotik.
Most of the times, the solution is to put a non managed (dumb L2) switch between the Mikrotik and the Airfibre. I know, you want as less as possible active components in your network, but this solves quickly the “problem”.