Not sure why, but when I remove your plugin, everything works fine.

Also, not sure if you noticed, but the gingerbread skinID and the Brutalist SkinID are the same (10225). So if you are checking that, make sure you know which block grade you are looking at.

Strange... I tried removing all my other grade plugins... and it still occurred. Once I removed yours it seemed to be fine... I will continue to poke around.

Explain in more detail how this happens and what plugins are installed on the server that change the buildings

BuildingSkins, BuildingGrades, & BGrade.

If I have deployed a building block with a gingerbread skin and then smack it with my hammer, it auto changes the skin to frontier. 

When I remove all the plugins but yours the behavior persists. Once I remove your BuildingSkins plugin, it behaves normally. I then add back the BuildingGrades and BGrade one at a time. Still no issues. I added your BuildingSkins back last, problem returns.

I doubt it's one of the other two plugins, seeing as neither of them call or use the OnHammerHit function.

I have both of these plugins installed to check compatibility (with standard settings) and I cannot detect such a problem.

Merged post

can you create a clean test server and install only these plugins and their dependencies there and test again?