I feel like people who play a lot would have noticed this right away but I don't see any threads on it so:
When I lose a t2 or t3 mex and attempt to rebuild a lower tier mex on the wreck, the wreck disappears when the builder attempts to build, giving no reclaim and stopping the builder. This happens almost every time.
To recreate: make a support commander and tell him to build a t2 mex. Ctrl-k the mex and tell him to build a t1 mex on the wreck. It should disappear when he attempts to build and he will then build nothing and stop, similar to the bug that occurs when you tell a large group of support commanders to build something and they skip over it. With the wreck of a t3 mex, attempting to build both t2 and t1 mexes on the wreck create the same bug. This is potentially over 3700 mass in reclaim just disappearing.
I use gaz and hotbuild; please some people attempt this to make sure it's not just my setup malfunctioning.