Hello ddrogin,
It sounds like customized alarms are not playing the selected song after updating iOS.
We'll be happy to help.
For the alarm to play the selected song, it will need to be downloaded to your iPhone. It's possible that the song in question is not downloaded to your iPhone.
When you have a free moment, try these steps and then test by setting up a new alarm: Add music to iPhone and listen offline.
*Download a song, album, or playlist: Touch and hold music you’ve added to your library, then tap Download.
After you’ve added a playlist or album, you can also tap
at the top of the screen.
Note: You must turn on Sync Library to download music from Apple Music to your library (go to Settings
> Music, then turn on Sync Library).
*Always download music: Go to Settings > Music, then turn on Automatic Downloads.
Songs you add are automatically downloaded to iPhone.
*See download progress: On the Library screen, tap Downloaded Music, then tap Downloading.
Note: If the music you’re downloading is available in Dolby Atmos, the Dolby button
appears next to the item, and you can download it either in Dolby Atmos or in stereo. To download music in Dolby Atmos when available, go to Settings
> Music, then turn on Download in Dolby Atmos. See Listen to Dolby Atmos music on iPhone.
How to set and change alarms on your iPhone - Apple Support
1) Open the Clock app, then tap the Alarm tab.
2) Tap the Add button
.
3) Set a time for the alarm. You can also choose one of these options:
*Repeat: Tap to set up a recurring alarm.
*Label: Tap to name your alarm.
*Sound: Tap to pick a sound that plays when the alarm sounds.
*Snooze: Turn on to see a Snooze option when the alarm sounds.
4) Tap Save.
We hope this helps.
If you’re still experiencing the same issue, reach back out to the community for more help.
Thanks for using the Apple Support Communities.
Cheers!