TBird for Android, v 8.1 OS 15, IMAP account set to fetch every 30 minutes. I got a normal notification on my phone about a new email. I check mail via Thunderbird on my Windows laptop, and delete the email when done. An hour later, the notification on the phone is still there and when I open the app, the email is still there, then it disappears after a few seconds as TBird checks for mail when the app is opened. The email and notification should have been removed from the phone within 30 minutes per my settings. Screen shot of the relevant settings page included.