I placed Condensed Explosives, broke it, and got Anvil Explosives.

Check to make sure the server and client both have the same data in config/icbmclassic/explosive_reg.json. As this could result in it showing visually as one block on the client and existing as a different block on the server.

Testing with latest I'm unable to replicate this issue so closing for now. Reopen with additional information such as specific version, other mods installed, logs, and video if possible.