Join a Chat Group Calll

Hi,

I have two group chats A and B

One we always leave a call turned on. And there is a join button, so we can leave the call, and join back again.

Group A

I don't know why this does not work with the other group, there is no 'Join' button despite both being exactly identical group chats.

I do get different menu icons not sure why.

Group B

How can I make group B behave like group A? It would be really practical to be able to join and leave calls on both groups without having people to invite you to the call.

I can't understand the discrepancy, both are Group Chats Calls.