It seems like the repair hammer will hit slightly beneath the actual aim indicator. This allows you to fully bury foundations, making them impossible to hit, and still be able to repair them from the outside. This happened a while ago and I just now came across the forums, so it may have been fixed. However, it seems easy to overlook and decided it would be best to report it here. I did report this in the “exploits” section of the steam forums, but thought it best to post it here as well.
You can also hit f7 ingame and use the report tool.