For what it’s worth, mattermost reset the config file back to defaults again after updating to 5.32.1 yesterday.
After the update I start sudo systemctl start mattermost without plugins so I can change/revert/save settings in the system console (as per upgrade instructions).
Before touching anything, I notice the banner “please set site name” (or something along those lines) at the top, indicating that the config file has been wiped. On inspection in the file system, I can confirm config.json has been reset to defaults.
I have merged the blank config created by mattermost with my previous file and it has been working since then.