Folder polling is not reliable any more

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App version

6.803 (still happening with Thunderbird for Android 22.0)

Where did you get the app from?

Google Play

Android version

14 (still happening with 17)

Device model

Pixel 7a

Steps to reproduce

1. Configure the app to poll for new emails every 15 minutes.
2. Turn off the phone screen and do not use the phone.
3. Send yourself an email from another device.
4. Wait for the notification.

Expected behavior

Notifications for new emails should be shown not more than 15 minutes later.

Actual behavior

No notifications for new emails are shown. As far I can tell it's not just that the notification is missing, but the app doesn't even poll for emails.

Sometimes I get the emails when I wake up the phone (turn the screen on).
Sometimes I need to unlock the phone to get them.
Sometimes I even need to open the app. In those cases I see that no new emails are shown, then the progress indicator line on top of the email list shows up, and only then the emails are polled.

This problem started a few weeks ago. Maybe with the update to version 6.802. Maybe it was also an Android update (e.g. April or May update) that caused it.

The Android setting/permission for battery usage of the app is already set to "allow background usage" and "not limited" (my translations, I'm not sure on the official english wording of those settings).

Logs

No response

No source file or test is named in the report. First reproduce the 15-minute polling scenario with the screen off, then trace the background polling and notification entry points for Android. Done means new-email notifications arrive within 15 minutes without waking, unlocking, or opening the app.