This size is a value read from the databse. When you just create a small text file in this folder, and then delete it again, it should recalculate the size.
Normally the size is updated automatically, if you delete a file just in the file system (not through Nextcloud), it is not aware of this change. Other reasons can be if some database operations did not finish properly.
Question is what happened to the 5th folder. Was it deleted by someone? Or a bug that made it disappear?
Are you really using 28.0.0? If so, your problem may just be being way out-of-date with maintenance releases. v28 is on 28.0.12 (soon to be 28.0.13) at this point.
This is true in general, but 28.0.0 in particular had massive front-end changes that took a couple maintenance releases to get fully formed.
I want to recover data from the 5th folder that was lost.
Anyhow, as to your original query: These folders are stored in data/__groupfolders/ on-disk. You can see what is still there.