https://community.sophos.com/sophos-xg-firewall/b/blog/posts/sophos-firewall-v18-5-mr3-is-now-available
Release Notes: https://docs.sophos.com/releasenotes/output/en-us/nsg/sf_185_rn.html
"Old" V18.5 MR2 Thread: https://community.sophos.com/sophos-xg-firewall/b/blog/posts/sophos-firewall-v18-5-mr3-is-now-available
After the update, we had problems with the IP address assignment from the DHCP server for requests via relay. After the rollback to 18.5.2 everything worked again.
did you try do delete and recreate a random DHCP relay object? We have this problem at random times since v18 and are currently on 18.5.2. every some months, all clients fail to get DHCP. Then we recreate a DHCP server or relay object on XG and all is working again. Sophos-style.