I can repro this. 
My step-by-step:
- Add
https://meta.discourse.org/c/bug/1.rssto RSS Polling - Set user and category, but leave tags blank (save)
- Wait for topics to be pulled in
- Select a couple and manually add tags
- Wait for the next poll
- See a topic with manually added tags has been edited to remove them
Expected: Polled topic with manually added tags should not be edited to remove those tags
Actual: Polled topic tags are overwritten
(No error in the /logs though)