I have been using MC14 for a long time and just installed MC 15. I have a variety of custom fields that are set for "store in file tags."
When MC15 is first installed, it launches the default library, which does not you have any user fields. It also immediately begins running auto-import and scanning all drives for files. It loads the found files into MC, and, since there are not yet any user defined fields, it removes that data from the tags of the files...the data in the custom fields stored in the files is lost.
I was aware that I needed to create a new library and add my user fields before I started importing my files. Unfortunately, MC never gave me a chance. All the processes launched automatically and it started destroying tags from the moment I first launched the program.
I may be able to recover from my MC14 library. Recommendations for the best way to do this? However it is set to watch folders for changes and to update the library for external changes. So, I'll have to be careful - have a backup of the library first. And I do have a backup of the files as a last resort. But I'm going to waste a lot of time (something I don't have at the moment) fixing this problem.
Also, I have that when I turn off auto-import and then upgrade to a new release of MC (ie. from 15.0.010 to 15.0.023), it turns auto-import back on.