> You are correct.  I used '+L' NOT '-L'.

I would add -nP -> "lsof -nP +L1"
If negative, I would go for the ugly path, grep'ing lsof's output on
"deleted" or "(deleted)".

Past this point, if space of alleged deleted files is not cleared... I
wonder. Even ext2 should do the trick.
If not ext4, I would upgrade to it by changing the flags with the help
of tune2fs.

I am especially surprises by the fact that you stated you could unmount
the volume and remount it, still without seeing free space
improvement... Are you positive you deleted the real files, and not mere
links to them? I would make sure by using du to seek for actual disk
usage location.

In a last, desperate resort, I would try to force allocation/release of
free space through sfill

Bernard (Beer) Rosset
https://rosset.net/