Sharing my ideas, do not assign specific uplinks and let the APs determine the best Root or next-hop AP to join.
Measure the signal strength of neighbor APs seen, from all your AP support info files.
Mesh APs require 25 RSSI in both directions to maintain mesh links.
It might be that MAP1 is/was on a bad channel for interference from your 2nd hop AP, so it wanted to go thru MAP2,
to maintain better throughput for clients beyond the 2nd hop AP.
If the MAP1 interference was temporary, the link with MAP2 would still have to drop to below minimum before your
2nd hop AP would likely switch back to MAP1 though.