DHCP option66 is just for setting the tftp server for the auto-provisioning of ip phones. Also this is used on dhcp server of another subnet, it is 100% not related. For the sake of info, the IP in the value is just the pbx IP.
I did mention earlier, the VLANs on this router are set up "the old way" where you create a separate bridge that includes the vlan interface along with all the interfaces that should be on same subnet.
I know it is dated, the reason being that this was setup more than 5 years ago, in RoutersOS v6. In reality, when there will be enough time available, I will update it to current way of implementing VLANs, but as a setup it works.
Also from AP standpoint, since it's still the OLD capsman, this is setup with capsman-forwarding so all the wifi related traffic of APs is forwarded directly to capsman through single VLAN.
The problem I describe in this post exists since some months ago, meaning for example that on 7.14.X version it didn't happen (or to be more precise, it wasn't noticeable if there was).
After I managed to pinpoint a bit better the problem, I found the following post which seems like the one I am facing. Hotspot and multiple IP's per MAC
Edit for clarification: This export I posted includes both capsman v1 and v2, since I have both mipsbe and arm64. The problem I face is related to capsman v1 and it is the same on another site with similar setup and only capsman v1.