Sure, though now some time has passed since I updated the snap. If I need to release a new snap again/do whatever else to reproduce this issue please let me know I can do that.
root:/# snap refresh --time
schedule: 3:00-5:00
last: 5 days ago, at 18:08 MSK
next: 5 days ago, at 07:47 MSK
Just for anyone having this issue… Looks like it was because snapd started using a new configuration option.
refresh.schedule: "3:00-5:00"
which I had in gadget.yaml does not work anymore.
refresh.timer needs to be used. Which is super frustrating to find out in this way. Hopefully all older devices won’t have a broken schedule for updates (does not seem to be the case for UC 16 at least).