Typing in the tag field will offer the matching results from previously used tags. If the user selects one of the offered tags (by mouse, by keyboard with space or enter), then the entry field should be cleared and activated for the entry of a new tag. One could have multiple tags for the same task. (I used the web interface for that)
Right now, after finding the right tag, I have to manually go back to the entry field, and delete the characters. I can’t think of a use case, where it is beneficial.