As I argued against the same in Google, post migration, closing all Threads from replies, I would feel the same here. Keep in mind I am coming from 10+ years experience with the current forum.
However if we feel we consider closing threads as a “best practice” and worthy of adoption I suggest clearly documenting this difference between forums and how to handle this change for all users. As you pointed out transcluding a thread.
There may need to be exceptions such as the recently discussed links from tiddlywiki.com into Discourse.
Finally this thread is a case in point, in some ways, I did not mark it solved but I did raise the question. What if I were away for a week I could come back and find it closed. One of the advantages of the never closing threads is they can continue over a long period, with no need for me to attend to tiddlywiki issues on regular basis. This aspect has helped the community members drop in and out of the community, as needed rather than be as engages as us enthusiasts.
Finally as stated before, if the solved settings exist only on a Q&A Category and so too does the auto close this may be OK, because people may expect a different behaviour in the Q&A but I would not like to see it on the discussion category.
Like here actually.
Yours Sincerely.