No notification on mentions or subscribed issues, merge request

We shortly upgraded to 8.0 and we just discovered that if one reference someone else with @ or if someone writes into a issue/merge request you've subscribed to no one gets a notification about that anymore. It is just not possible to get this notifications. This is a major bug as it breaks the whole communication. All other notifications like getting assigned to a merge request and so on are working.

Is there maybe any configuration option I haven't found yet or is this a bug?

Thank you