I've had similar issues both with WDS (on separate SSID) and mesh11sd. I need either one to connect one of my AP's wirelessly (I don't have the possibility to wire it at that location).
After lots of testing it seems to do with the handling of multiple SSID's (which is also needed for 802.11s). This breaks the connection after several hours/days.
I've changed to a setup where the same SSID is used for clients and for the WDS connection to my other AP. Now, my connection is completely stable for about 3 months without any hiccup.
Maybe a driver issue in the support of multiple AP's on the same band?