" Learning was set to “no”. Don’t remember doing that in this home testlab , but it was there."
Forum knows 
, last year …
http://forum.mikrotik.com/t/tp-link-repeater-re450-not-assigning-ip-address/118970/1
Edit: To be clear, setting the “bridge port” learn mode to “auto” solved everything. That was the modification I made last year, and hit me with mysterious disconnects a year later (I had not seen it all that time, because I had been using other virtual WLAN interfaces testing and using the new RADIUS/User Manager login.)