Issue notification is not working for specific projects

System Version: self-managed Gitlab instance with version 14.2.4-ee.

We have a strange problem and I have neither any idea what the problem could be nor where to start searching for the reason.
In one of our projects, the participants of an issue do not get an email notification when a colleague writes a comment to an issue including an attachment!
If a colleague writes a comment without any attachment, then all issue-participants get an email notification.

Does anybody has any idea what the problem could be? or at least any idea where I could start to search for the problem?

Many thanks in advance!