I have a mesh topology with 1510 and 1522 Access Points, 18 int the total. In my headquarter I have two RAPs, one 1510 and other 1522. They are the distance between 7 meters. The others are MAPs. All of then in the same bridge group name.
I notice that the MAPs prefer the 1510 to be a parent.
No problem at all with later versions of code. I am interested to discover what your channel settings on the RAPs. I wouldn't put two that close together unless they were sectorized and on different channels. The algorithm for determining which parent to attach to in the Adaptive Wireless Protocol could get confused easily here and cause flapping to occur.