This isn’t an SEO no-no. You’re presenting an incredibly contrived set of circumstances. If you’re worried that TL3 users will do this, make TL3 unobtainable on your particular instance.
This also isn’t a bug, the redirect and slug updates aren’t a fluke, it was engineered to behave this way.
A bug report means something is broken, preventing normal/typical use of Discourse.
Which clearly isn’t true for the above.