I just tested this and all worked out. I added Morgan and Alex to the conversation and then after that added Reece. Reece could see the whole history in his Teams app and here is the screenshot from the desktop app on his machine:
However, this is users within the same tenant. Is the user within the same tenant or are they a guest?
Yeah! It seems it’s not the uservoice for this exact feature! I get that’s it’s more complicated due to things change when an external user is involved when chat setup is already established I’ll be at the AMA so I’ll see you there!