I recently discovered this as well. A bit unnerving. I now use Cryptomator (because key destruction matters).

I mean, you didn’t give it enough time. All of these cloud storage platforms are databases at their core. When you delete the file you’re updating the database entry, the data (and the record of it) is still there until their purge process runs, which could be days or weeks.

If it’s still there at a month I’d be surprised and be checking terms of service to see what they commit to.