I've recently started using GrapheneOS on a new Pixel 7a and since then had the problem that occasionally calendar reminders (I estimate less than 20%) are very late (on the scale of hours) or do not fire at all.
I'm using multiple calendars synced via DAVx5 and I have tried several calendar apps. I had Etar and aCalendar+ installed at the same time to check whether it is a bug in the app, but reminders always either fire for both apps at the same time or don't show up at all. I also tried the Fossify Calendar, which seems to use a different mechanism to display reminders. It works less accurately timing wise but seems to be 100% reliable. This app unfortunately does not have enough features to be usable for me on a daily basis.
When I remove a calendar in DAVx5 and re-sync, the missing reminders fire instantly, but this is not a solution. It does not improve the situation for future reminders. My calendars have multiple reminders per day.
My current workaround is to keep Fossify Calendar installed only to show reminders I may otherwise miss.
Has anyone seen something similar? Does anyone have any suggestions on how to debug this further?