Proton Mail not showing notifications

Affected app
Name: proton
Package id: ch.protonmail.android

Describe the bug
Not receiving notifications for new incoming emails (or any other notifications.

To Reproduce

1. Send an email to yourself (from another email address)
2. Wait for a while and see no notification

Expected behavior
To get a notification for a new email.

System
Android Version: 16
Custom ROM: LineageOS 23.1

microG
microG Core version: 0.3.12.250932
microG Self-Check results: All ticked except for MicroG Services Framework installed, Permission to receive text messages, Permission to This app can appear on top of other apps, Permission to interact with work profile. I'm unable to tick them.

Additional context
Xiaomi Mi 11 (Venus) phone, with [ROM][Android 16][UNOFFICIAL][OTA] LineageOS 23.1 for Xiaomi Mi 11 [venus].

This ROM doesn't support MicroG specifically.

Is there a way for me to get MicroG compatible with my device? I'm a programmer but not in the area of phones.

Everything else on my phone seems to work fine except for this issue. Of course I would love to help fixing this issue.

The issue names no source files or tests. Start by reproducing the missing Proton Mail notifications on the listed Xiaomi Mi 11, LineageOS 23.1, Android 16 and microG setup, then inspect the notification or push-delivery path involved. Done means incoming Proton Mail messages and other notifications appear reliably.