I don't plan to leave it like this, I just needed a third node and a replacement system was supplied with Proxmox 9 on it already - was running out of time so just tried a join and it has worked OK so far. I will upgrade the remaining nodes over...
Just in case anyone is curious, I added a Proxmox 9.1 host to an 8.4 cluster without issues today. I even configured replication, as the plan is to upgrade an existing cluster to 9.1 and use this as a destinations to VMs to run while hosts are...