I have the same issue as this post. And the solution here https://meta.discourse.org/t/how-do-i-disable-post-auto-close/109554/6?u=taher1 doesn’t work anymore, upon hitting save after clearing the number field it says Auto close hours is not a number.
Sorry, can you post a screenshot? That old topic was not helpful.
Once you set Auto-close topic after in a category, it appears that you can’t undo this setting (remove automatic topic timers).
Probably a new bug as @martin is currently working in this area.
I have a fix merged for this now, the issue was that the new control used to input the hours did not handle blank values: