Hi @User28375662, welcome to the Sonos Community!

It does sound like an issue with the alarm itself but it’s a bit strange as alarms default to no limit under duration. I’d delete the alarm to start with then try the following:

First I’d suggest trying to create a new alarm for a different time with no limit to see if that also stops after one hour. While this step sounds counter intuitive, I’d also suggest setting a time limit on the alarm and make it longer than one hour (you can set it up to the max of 23 hours 59 minutes if you want). This would be good to see if the alarm still cuts after one hour even when a longer time limit is set rather than no time limit.

If you have another controller with the Sonos app installed then you could also try setup the alarm using that.

I hope the steps above help, but if they don’t, then please replicate the issue and submit a diagnostics after the alarm stops playing.