db_delete does not actually delete
db_delete works fine, but the problem is that I was trying to delete the cache of the current form from within the code of that form e.g. submit form. Logically this is ok for me because I was trying to delete the cache at the end of my multistep form where the user cannot do anything else but navigate away.
There must be a check somewhere either in db_delete not to allow to delete the cache of the current form.
I tried and I managed to delete from cache_form other entries from the exact same place in the code, just not targeting the current_form cache.
I was aware that I'm deleting the cache of the current form, but I just wanted to test that the db_delete works, and then move on with a more complicated logic where I would not delete the current cache but the previous form from the cache.