This is a bad cloud issue of some sort. I have had this issue since 2016 and it just comes back and forth. An Apple rep will say restore and set up as new but that just fixes it for temporarily. Since the issue is cloud related in some sort. If your using custom sounds like Apple Music then that is the root issue I have found on my end so I just use the default alarm sound and since then no issues have came back.
This issue has came back for me on 4 different iPhones and I have never restored from a backup on any of them, I always set up as new as I have learned while working in IT it is better to let data sync naturally from a cloud server vs restoring from a backup that was made on totally different hardware/architecture. I attempted multiple set ups as new hoping to avoid the issue if it was some how part of a data corruption on the OS or the backup it’s self but still no luck. And yes the alarms do sync via cloud as confirmed with engineering at Apple as you can see cloud messages when attempting to turn on alarms on items such as HomePods. Apple just hasn’t developed a permanent fix for this particular issue from what it sounds like, I haven’t trusted using music for alarms since around 2018 when I was tired of the issue randomly coming back.They do release updates to fix the issue but where it is cloud related it just pivots back to a old database server and then Issue comes back. 9 years I’ve had that same issue come back and forth. Not too mention on my wife’s device her find my shows my old Apple ID which also from 2016, which further points to a bad cloud corruption issue in there cloud server. (My old Apple account ID should never show up as it technically doesn’t exist anymore since I fully changed the email fully for the Apple account and no the old Apple account t email is not used as a alias/secondary account email).
My next step of isolation is fully restoring my iPhone 14 Pro and making a new Apple account just to prove the point it’s linked to my Apple account :)
Good luck with it! Remember if your using music for alarms try the default tones like the “alarm” option and see if the issue replicates for you.