Do a Get Info on the drive and look for "Purgeable". Many times when macOS clears large chunks of data, the space becomes "Purgeable". Apple says it's usable as needed, but it does seem to take from a few minutes to days for it to be returned to useable space.
If you have Purgeable that matches the amount of space you "freed up" you'll need to wait for it to be returned to available space. Or you can use an app like the paid version of DaisyDisk. DD is cheap, free, safe and does a good job of returning Purgeable space to usefulness.
If Purgeable is not the problem, more investigation is needed.