The alarm should continue to function, surviving a power cycle. It shouldn’t survive a factory reset, as the data is stored on the speaker itself, and not in the controller application. I would recommend that you submit two system diagnostics, one with the alarm set/working, and then another after ‘erasing’ the alarm by a power cycle, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.