Can’t replicate it in a test Discourse instance ( v1.8.0.beta4 +27 ) - but some accounts appear to create notifications for the @mentioned account properly, some accounts appear not to do so.
A specific case: my @mentioning the @kassandra account does not show up in the notification array and vice versa. Some accounts do appear to create @mentions properly for both these accounts (e.g. @DSV_Coleridge)
In situ example: