Hello there,
I am part of the plentymarkets forum. Meanwhile also as an employee of plentymarkets. I have noticed that the subscription function does not react when a tag is inserted into a thread afterwards. It only reacts if the tag is set when creating the thread.
But I would also like to see the topics that are tagged later. What can I do?
best regards
Daniel
Friendly bump 
I can confirm that adding a tag after topic creation doesn’t trigger any notifications for watched tags. I’m moving this to the #bug category.
Hello @gerhard,
thank you for changing the category. As i have noticed, there is also the same bug when changing the category afterwards
It’s working for category changes in my tests with the latest version of Discourse.
No repro. Adding a tag to an existing topic successfully triggers a notification for a user watching the tag.