Deleted files don't free up space

I have a USB thumb drive, formatted in APFS encrypted. I use it to store backups of family records. The other day I deleted a number of files as the drive was getting pretty full. To my surprise I noticed that according to "Get Info", there is no more room on the drive after deleting the files (and emptying the trash) than there was before I did the deletions. How could this be? Is it because the drive is encrypted? Is there any way to get my space back?