Hi,
Regardless of version, APs dont function without controllers so in your case you are definitely running an IAP.
The new link wont appear if auto join is disabled. By default its disabled.
Need to check the following
1) Can you check if the old IAP is configured as standalone? in which case it will not (and wont let others) join.
If above is not the case, can you check below
1) You said all new aps start by broadcasting setmeup ssid, does it mean they have formed their own cluster?. can you check by connecting to this ssid and see if all APs (except the old one) are showing under access point column?
2) The newer APs may have been shipped probably with a newer version. That should still not allow them to form a separate cluster.