New email not shown in list, only when refreshing full page

Steps to reproduce

1. Open mail in firefox (linux / windows both have the same symptom)
2. New email is coming in and can be opened
3. After some time new email is show where now a zero is shown and is not shown in the list of emails

Only after refreshing the page the new email is shown.
As this appears random, it is difficult to troubleshoot.
Side question, where is the number 8 pointing to?

Thanks for helping with this one too (posted a few more today, based on longer experience)!

Expected behavior

The refresh button inthe middle should show the email:

Actual behavior

No new email is shown

Mail app version

mail - settings about should contain the version number, it is 5.6.4 (current)

Nextcloud version

25 autumn (earlier versions have shown this too)

Mailserver or service

mailcow (suspect this is not the cause)

Operating system

Linux and windows in Firefox, running aio in the backend on linux

PHP engine version

None

Nextcloud memory caching

No response

Web server

None

Database

None

Additional info

No response

Start by reproducing the issue in Firefox on Linux or Windows with Mail app 5.6.4 and Nextcloud 25, focusing on the refresh button and email list after a new message arrives. Compare the partial refresh with a full page refresh; done means the new email appears in the list without reloading the entire page.