If it’s playing the chime, rather than the set alarm ‘station’, that’s a good indication that at that exact ‘moment’, the Sonos is unable to reach the station. Might be local wifi interference , might be that the station is overwhelmed with concurrent requests for new streams. Rather than not play anything, and you missing the alarm, Sonos chooses to play the chime.

You might try a couple of things. First, move the alarm earlier by a minute or two (or three, or four….) to see if it’s their server being overwhelmed by requests for data. If that doesn’t work, I’d try setting up a ‘secondary’ alarm for five minutes earlier, with a zero volume output for that station, to ‘waken’ your local LAN and make sure it’s ready for the alarm command.

Of course, there’s also the idea that you could call Sonos Support directly to discuss it. If you submit a diagnostic within 10 minutes of the alarm ‘failing’, they’d have some hard data to look at. 

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 Sonos system and network.