Impossible to add an existing tag if it is not used elsewhere

Hi guys,
My scenario is:

  1. I am creating a tag from the Task view
  2. It is possible now to add this tag to other tasks - tags are searched correctly when I start to type the name of the tag.
  3. I am removing tag-task associations.
  4. When the tag is not used elsewhere (is not associated with any task) it starts to be impossible to add this tag to tasks - when I type the name system want to create a new tag with the same name.
  5. I found the workaround - if I know id of the tag, I could go to the list of the connected tasks and there the option “Add task via email” is available. When I create tasks from an email it starts to be possible to add the tag to other tasks.

In my opinion, the problem is in searching functionality. The system returns only tags connected to tasks, but I am not sure it is correct behaviour.