Published app doesn't update or install on some devices

I started getting a lot of reports that my "Universal App" doesn't update for some users. If they delete it, they can't install it back. The app doesn't use Garmin Pay.

I updated the app with the most recent SDK and published an update - it was accepted without any issues. However, when I tried updating it on my watches, one of the watches had exactly the same issue as other users. On Fenix 8 AMOLED, the update was queued forever. When I uninstalled, I couldn't install it back - it would say "installing", then briefly "settings", then back to "install". In log file I found a bunch of these:

---
Error: 'Signature check failed on file: DB1A9BFA'
Time: 2026-05-03T01:40:38Z
Part-Number: 006-B4536-00
Firmware-Version: '21.39'
Language-Code: eng

What is weird, is that this app updated on my other watch Enduro 3 without any problems. Same user account.

Same thing happens to my other app (HYDRATE+), except that watches switched (installs on Fenix 8, but not on Enduro 3). Users complaining too:

The Connect IQ user A Mr has sent you the following request regarding your app "HYDRATE+":

hi, I can't get it installed. IQ just says installing and then the button us reseted. any suggestions?
THX!