I know this isn’t going to be the answer you are looking for, unfortunately when that happens, I do not believe there is a way to get it back.
Having the entire sim on a custom drive really won’t prevent this from happening.
If you do decide to join again, the recommended way to protect your personal content is: (this is from Beta Testers: New Information on Joining/Leaving the Beta
" We recommend that those who wish to test with us ensure your sim content is located in a custom path before joining the flight to avoid full reinstallations in the future (Details on how to do this can be found here ). The best set up would be for your store app to live on the default C: drive and sim content on a custom drive."
How to move your sim content (the found here link) is this post:
Rebuilding your content may be the only solution.