Deleted You are correct it is not a Forms-specific issue in fact it is not a Forms issue at all and more an MS Teams integration issue wherein On-Demand meetings it is enabled and in scheduled meetings, it is not.
The difference between how the Meeting is started is a matter of time and in regards to the way, Microsoft Forms works in Channels that is someone's coding that made it feed it into the channel so it has been specifically coded to do that INSTEAD of how it works in non-channel meetings.
What would be nice is having a choice that could be applied within the meeting options or on the Teams Admin switching between one or the other that best suits your business needs instead of having the choice made for us and ending up with a ton of features that are utterly useless because of a choice we didn't make.