Then an option to do a backup should have been presented. I would suggest that is a bug.
I have similar issues. Was running 20w51a. 1.16.5-rc1 updated with no chance for backup. It did say version was older but not sure how. Lost all copper other items.
RC2 downloaded and now I'm not able to recreated. This can be close. Thanks.
GTX970 with Fabulous here as well. Changed to "fancy" and they disappear.
There was no option to backup. That is a bug.