Yes! I also got bitten by format_on_save when working with legacy projects with inconsistent formatting. Given another discussion I saw, the maintainers didn't think about this use case much, i.e. "why don't you want to have a proper formatting?". It can be turned off now, so not sure if they'll change the default.
It's like Excel changing .csv files after opening them so a simple load/save cycle can corrupt the file and your original copy is nowhere to be found.
I imagine the damage is smaller in case of auto-formatter but still - not something I would expect a program to do to my file in a simple "open file - close the program" cycle.
