I don’t really know what your problem is. This is how I would test. I would create a new field group and save it to see if the field name is getting changed. If it is I would start deactivating plugins and keep testing until the problem goes away. My first suspects would be plugins that are out of date or that are supposed to work with ACF. If deactivated all the plugins I would switch themes and see if something there was causing it. I would probably set up a duplicated of the site on a dev/testing server and to the debugging there.
