Disk space not freed up after deleting files

Did you empty the Trash after deleting the files on the flash drive? They will be left in the Trash, still "counting" against the drive storage on the flash drive until emptying the trash.

After deleting the files, did you also empty the bin?

I find it helpful to view the trash files by volume. That would make it clear whether the files are in Trash.

Click on Trash and then set the Finder View to Use Groups and then select Volume in Group By.

You could then select only the external files and right-click to Delete Immediately if you did not want to delete your other trash.

Refer to Apple Support Documentation on How to Erase and External Drive

The very important caveat is in the below Image from that same Link

link below

https://support.apple.com/en-ca/guide/disk-utility/dskutl14079/22.0/mac/13.0

On all my external drives, if I move a file to the Trash, it is visible there until the Trash is emptied, just as with my internal drive. So something may be amiss with your flash drive, or maybe you emptied the Trash already, or maybe it has a non-Apple file system. Or of course if you erased the drive, any of its files that were in the Trash will also be gone.

"toggle on the file directory" is not how file systems work in MacOS nor under other operating systems. When something is deleted it is moved in the file system from its old folder to a new folder, the Trash folder. The file system is like a directory, it tells where to find that file.

It is not a design flaw -- the purpose of putting deleted files in the Trash folder is so you can retrieve them if you decide later that deleting them is not what you wanted. If the physical space were made available immediately and were overwritten with new files, the file in the Trash would be irretrievable. Other OS work this way also, not just MacOS.