I have noticed the the quick action "Clear tags" doesn't always clear all tags from a task. Right now I had a task with two tags and the clear tag quick action only cleared one of them regardless how many times I clicked. Had to manually delete the tag.
Ok, I'm learning
It was "old" tags without a quick action assigned.
The "clear tags" quick action only works for tags that have quick action assigned.
Yes, you're right. Clear tag clears tags that's used in Quick Actions or Smart list filters. You can check the tags by editing 'Clear Tag' quick action.