What method of authentication are you using? WPA3-SAE, WPA3-Enterprise, something else?
Is the management interface of both APs in the same VLAN (they need to exchange roaming info)? Make sure you are not using any switch security features that restrict traffic between the ports (such as isolated ports).
Are you running current stable or better firmware on the APs?
Is your test client using up-to-date WiFi drivers?