a partition with missing files, but space and inodes were used

a had a partition on one of my disks that was formatted a few months ago in Xubuntu 18.04. i mounted it on 20.04 but never did anything with it there. i did a clean shutdown which i presume did a clean unmount. back on 18.04 i was going to add more files to it but rsync complained of a missing directory. checking the apex directory, it was totally empty. even lost+found was missing. df showed space used and df -i showed inodes used. i unmounted it and ran e2fsck on it and it indicated it was OK. then i ran e2fsck -f and it took a while and gave no error messages. i mounted it back and everything was back. did e2fsck do that or did remounting do that? what could have caused this (i have never seen such a thing before)?

i should have done a plain mount but i was afraid that night totally lose files. it was one of three backup disks, but it didn't want to have to write it all, again. it would not have lost data but it would have lost time. i would have wanted to get back to three backups.