Why notification toggle on sidebar for issues sometimes can't be enabled

I don’t understand why I can subscribe to some issues and can’t to others using Notification toggle on sidebar.

For example, I can enable Notification for this issue:

And I can’t enable Notification for this issue:

Hi @AndreySV and welcome to the GitLab Community forum! :tada:

If you can’t enable notifications, that means the project owner has disabled email notifications be checkiing the “Disable email notifications” in the Project settings.

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Thank you for the answer. I’ve asked project’s owner about this settings (Mobian · GitLab) and he told me that “Disable email notifications” option is not set.
I’ve created issue in my own project (Disable email notifications is not set):
test issue (#1) · Issues · Andrey Skvortsov / mutt · GitLab

and tried to enable notification for another created test account. Under test account I can’t enable Notifications for the issue (subscribe to updates). Slider is blocked.

This options is disabled like on your screenshot. Are there any other options that could cause this?