I think this happens when a tag that is not visible to the user was added while there is also a visible tag.
I can reproduce with the following steps:
Use a test user to create a new topic and post the new topic.
Use an admin (or another user with permission to add tags) and add at least one tag to the topic that is not in a tag group with limited visibility and at least one tag that the test user cannot see. Save the changes.
As the test user, go to your topic, click the pencil to edit, and add a new line to the body.
Expected result: No edit conflict, as no one else is currently editing the topic.
Actual result: The conflict indicator is shown.
Additional note: It does not matter who adds the visible tag. For example, it also works when the test user adds it before creating the topic.