I can’t, as an admin, change ownership of a topic. And safe-mode didn’t bring any clearence. I’m on iPad, so can’t tell if there is any errors in console. Diacourse is upgraded about an hour ago. What else… in that category is no unusually limitations, visible for everyone and writing is for TL1 — but I tried change ownership to staff and ordinary TL1 without success.
AI-generated summary
Jagster reported an issue where, as an admin, they were unable to change ownership of a topic on their Discourse site, despite having no unusual limitations and the topic being visible to everyone. JammyDodger tested the issue and found it worked on their test site, but suspected the issue might be related to the ActivityPub plugin. Jagster confirmed the plugin was active on the topic’s category. JammyDodger explained that this is a current limitation of the ActivityPub plugin, which can be puzzling until recognized. Jagster noted that while they understand the limitation for the first post, they are concerned that it affects all posts under the topic, which should not be seen by ActivityPub at the moment. A related topic on the ActivityPub plugin can be found here.
I’ve just had a test run and it worked for me on my test site. I could successfully change an OP to a different user.
By any chance, do you have the ActivityPub plugin active on that topic’s category?
Oh damn 
Of course. And yes I have.
That caught me out for some time the first time I encountered it in our Contribute > Feature category. 
It’s a current limitation(/feature?) of the ActivityPub plugin, though can be a real puzzler until you click what’s going on.
I kind of understand it with the very first post, because it is delivered. But I can’t change ownership of any posts under such topic and that bothers me a bit because ActivityPub should not see those - at the moment anyway.
(Let’ connect topics together: ActivityPub Plugin )