The Auto-assign task "cannot be manually added" field does not work as expected. The purpose of the field, if set to "false", was to disallow the manual adding of the Auto-assigned tag, it instead:
1. Disallows the saving of manually added Tags to Tasks with auto-assigned Tags
2. Allows the auto-assigned Tag to be added ad hoc
When attempting to save manually added Tags to Tasks with existing Auto-assigned Tags, whose "Can be manually added" flag is set to "False", the following occurs:
1. The error message "Tag "x" cannot be manually added. It is an auto-assigned tag".
2. The Tag seems to have save and appears on the Task Panel, however when reopening the Task's "Edit Task" screen the manually added Tag will be removed from the Tag field.
3. If a Task already has a manual Tag added and an auto-assigned tag is added afterwards, when opening the Task's "Edit Task" screen the manually added Tag will be removed from the Tag field.
The work around for the issue, to remove the conflict between manually added Tags and auto-assigned Tags is not to use the "cannot be manually added" boolean.
We are currently working towards a permanent solution. If you have any questions please do not hesitate to contact Support.