[2021] User_empire_designs.txt suddenly deleted? [MacOS}

I don't know how or why editing one empire caused the loss of others in that same file, I'm sorry to say! Nothing makes any sense, even a game bug really.

Can you repro the issue with those steps, are you saying?  

Please do - after all this happened on your machine not mine! Obviously keep backups of the files you edit just in case.