[Bug] No DM Notifications when both Desktop and Mobile Preferences is set to Mentions

Description:

I already found some issue with Notification set to Mentions in #17689
But now we did all tests and can tell for sure, that there is a bug - No DM Notifications when both Desktop and Mobile Preferences is set to Mentions

Steps to reproduce:

1. User1 set Show Desktop Notifications For to Mentions
2. User1 set Push Mobile Notifications For to Mentions
3. User2 write DM to User1
4. User1 gets no notifications neither desktop or mobile

Expected behavior:

There should be notifications (both desktop and mobile, if desktop not online) on DM messages

Actual behavior:

Here is a problematic settings:

Here is a table (matrix) of all available and tested variants of types of chats and as a result - notifications.

If YES - there was notification
If NO - there wasn't notification
Filled in red - problem notifications. There should be notifications according to settings and other tests, but in fact - no notifications.

Server Setup Information:

• Version of Rocket.Chat Server: 3.1.1
• Operating System: CentOS7
• Deployment Method: docker
• Number of Running Instances: 25
• DB Replicaset Oplog: Enabled
• NodeJS Version: 12.16.1
• MongoDB Version: 4.0.17

Client Setup Information

• Desktop App or Browser Version: 2.17.9 and Chrome 83
• Operating System: Windows 10