If any of those are android users, it may be down to the non-android standards friendly battery optimisation the manufacturer ship their devices with (e.g. Samsung, Huawei, OnePlus, Xiamoi, etc.)
I've had more issues with missing notifications on my OnePlus than I ever had on my Pixel (I have list of reasons why I'll never buy Google hardware again, mostly coming down to their support). Most issues get fixed by me persuading OnePlus to stop applying their secret sauce to power control
Not disagreeing you, just adding a point...I self-host Conduit and use FluffyChat on my phone, with ntfy for push notifications. While that works great most of the time, I occasionally have to restrict ntfy because it'll get into some state where it destroys my battery... And push notifications obviously don't work at all in that state.
It's very unfortunate there isn't a privacy-friendly push notification mechanism built into Android that doesn't ruin battery life. Or iOS for that matter; that might actually be a feature that would get me to switch.
https://dontkillmyapp.com/
I've had more issues with missing notifications on my OnePlus than I ever had on my Pixel (I have list of reasons why I'll never buy Google hardware again, mostly coming down to their support). Most issues get fixed by me persuading OnePlus to stop applying their secret sauce to power control