Nexcloud Mail app still marks most incoming messages as important in Thunderbird email client, even when set (locally and globally) not to do that

Steps to reproduce

1. Under Nextcloud Admin Settings, ensure that "Enable classification of important mails by default" (under Groupware - Enable classification by importance by default) is toggled to the off position
2. In the Mail settings of the Nextcloud Mail app, under "Assistance features", ensure that "Mark as important" is unchecked.
3. Open Mozilla Thunderbird email client and check email for address linked to Nextcloud account

Expected behavior

No incoming emails should be automatically tagged as Important

Actual behavior

Most incoming emails are tagged as Important

Mail app version

5.1.0

Nextcloud version

30.0.10 (Nextcloud Hub 9)

Mailserver or service

No response

Operating system

Linux

PHP engine version

None

Nextcloud memory caching

No response

Web server

Other

Database

None

Additional info

This issue is driving me nuts to the point where I am thinking of cancelling my Nextcloud instance with Hetzner! It started without the Nexcloud Mail app even being installed (but with the global setting toggled to not allow automatic tagging of emails as important), so I installed / enabled the Mail app to change the relevant setting within the app but it made no difference. From what I have read, this issue seems to affect and cause a lot of frustration for many Nextcloud users.

Reproduce the behavior with Nextcloud 30.0.10, Mail 5.1.0, the global classification setting disabled, and the Mail app setting unchecked, then fetch mail through Mozilla Thunderbird. Trace where incoming messages are marked Important and verify that messages remain untagged when both settings are off.