Hi Lisa9,
welcome to the Apple (user-to-user) Community.
Re: "Issue with Alarm clock playing default sound"
Seems, when setting the alarm sound in the clock app, after you select the song you want, it is necessary to click "Save".
Set an alarm on iPhone – Apple Support
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Else, if it is the "wake-up alarm in your sleep schedule" ... seems you would need to choose a ringtone. (ie: a song is too long to be used ... ring-tones are much shorter.)
Change the next wake up alarm on iPhone – Apple Support
If you wish:
You may be able to make a ringtone from the song you want (eg: in Garageband)
Share songs with GarageBand for iPhone – Apple Support
All the best :-)