I have 3 chats.
The first one I cannot open directly (by tapping on it) in iOS 14.5.1.
I have to go to Contacts, find the contact, open Contact, and tap on
Send message which opens existing chat with that contact.
The rest (chat 2 and 3) I can select and maximize (open) chats.
I can’t see any pattern or uniqueness to that one chat that can’t be opened directly - I tried 5 times and ever time I was unable to open it directly, but able to open it indirectly, and minutes later (I haven’t restarted or reconnected) the problem was gone and all chats started behaving the same.