I have the same problem a records are ok (point for serwer work)... I got lets encrypt certificate, but subdomain site doesntt work, i get
"Service Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later."
whatever if i add it by subdomain or by domain writing that subdomain as domain.
I checked inside /public_html and outside - still the same problem.
If i add another domain similar way it work as should. Reuild vhosts or change webserwer to simplest apache or nginx doesnt work.
Its deafault config cwp pro for user/domain - it not work... (i have force php-fpm).
If it help - Almalinux 8.8 and newest cwp pro.
Server isnt overloaded in any way - have a lot of free ram, cpu and disk space.