No notifications are sent when user is mented with @

[nannielf]

If a user is relevant to record (f.e. he is "assigned to" or request approver), menting him with @ will nor trigger a notification. 

I can't find conditions for this. Is it OOB behaviour and is there any way to change it?

[nannielf]

Please, notice: problem affects ONLY users related to the record. 

Third party users receive an email after they have been mented, but neither request approver, nor the 'assigned to' person receive email notification after @menting . 

It's common behavior in all record types, regardless to the settings of specific users.