I don’t see anything pointing at domain.com (or any of its subdomains) in the source code of Syncthing. I also did a DNS lookup of all the fallback STUN servers, none were CNAMEs to domain.com either.
Some (stun.counterpath.net, stun.ekiga.net, stun.xten.com) were CNAMEs to stun.counterpath.com though.
SRV look-ups to _stun._udp.syncthing.net (this is hardcoded and not derived from your configured discovery servers), and look-ups to the associated A record (fyc5mja4mz5s0vmz1txx.syncthing.net atm, but the point of the SRV record is that this can change), are normal and expected on Syncthing 1.29.6 and newer.
Are you running an unofficial and/or outdated build of Syncthing by any chance?