Heartbeat authentication domain not found

I have a similar issue to the following discussion:

https://community.sophos.com/sophos-xg-firewall/f/discussions/127398/heartbeat-authentication-failed-username-without-domain-name-maybe-the-cause--live-users-disappear-but-still-connected-to-vpn

Access_server.log is showing [ADS_AUTH]: adsauth_handle_attrrequest: domain name 'clientdomain.com' not found. I have added multiple AD servers in XG authentication section with different domain names that are used in AD as UPN, 'clientdomain.com' being on one of them.

XG is on v18 MR5