Yes, they're on topic here. The current policy as stated in the help center and close reason does not make an exception, and there's not enough reason to change that policy. So for example:
Why did Experiment One not get a counter when my Undying creature returned to play? (it was a bug)
In Lords of Waterdeep on iOS, why does ...
It has always been the philosophy of the StackExchange network that determining whether a topic is on topic should be done on a purely per-site basis, without considering whether the topic is on topic on any other site. This means that it is OK if some questions are allowed on multiple sites, and it is also OK if some questions are not allowed on any site.