MT routing + Proxy Arp Bridge, Hotspot

Ive a little issue that just does not seam right (and it only exhibits with these two units, out of a few dozen).

Ive got a RB133 (2.9.46) with 3 cards in it (all Bh’s to hotspots) and a P3 1ghz (2.9.40)

I have a dedicated link to the tower, the radios do proxy arp. At the end of this link as another MT. The only the IPs on the link are the 2 radios and each MT. Both MT’s can ping each other (and both radios) but when told to route via MT1, it fails (all other traffic looks fine). Using a EoIP tunnel between MT1/MT2 and it works fine. Using a VAP on one of the grids the same need for a tunnel applies when proxy arp is happening (WDS solves it). Any ideas what could cause this? I plan to change the BH to a WDS link but I would still like to understand what is happening.