Hey ,
developers looked into it and unfortunately are not going to be able to fix it. In bug report, it was stated:
axis lock option uses constraints
to lock, and due to way solver
works, there can sometimes be slight
violation of constraints. Maybe in
future PhysX will give us another way
to fix this, but right now there isn’t
much we can do…
So you’ll need to stick with making sure your sprites are separated on Y by a large enough amount, as mentioned above.
For new issue you’ve discovered, it does appear to be related, but I believe fix will be similar. What is Collision Thickness on your floor and boulder sprites? default is 100, but if I recall correctly yours is much smaller. I tested this all way down to a Thickness of 20 with no problem. When I tried 10, however, I encountered same problem. If you raise boulder and/or floor sprites, do you have same issue? I would expect raising collision thickness on boulder should be enough.