Rocket.Chat stability issue when many users disconnected and connected again

Description:

Hi,
From time to time we have observing really serious Rocket.Chat stability issues. Duo to network problem, about half of our 3000 connected users are disconnected and connected again to Rocket.Chat. This situation caused under 100% processors load on Rocket docker containers and unavailability (messages greyed out, client disconnects). Also high containers RAM and Garbage Collector usage (screens below). It was probably caused by presence broadcast storm, because when we switched on "Disable Presence Broadcast", after couple of minutes everything back to normal. Switch off it again caused above problems and unavailability.
Permanent switch on "Disable Presence Broadcast" is not an option because user presence is not correct.

Steps to reproduce:

1. Server with about 3000 users online
2. In example restart reverse proxy (HAproxy, Nginx...)
3. Users are disconnected and connected again
4. Rocket.Chat is unavailable due to high load

Expected behavior:

Disconnect and connect again a lot of users same time not cause Rocket unavailable

Actual behavior:

Rocket is unavailable

Server Setup Information:

• Version of Rocket.Chat Server: 3.9.7
• Operating System: Centos7
• Deployment Method: docker-compose
• Number of Running Instances: 33
• DB Replicaset Oplog: YES
• NodeJS Version: v12.18.4
• MongoDB Version: 4.0.14

Client Setup Information

• Desktop App or Browser Version: All
• Operating System: All

Additional context

Docker contaniers CPU usage

Users sessions

Garbage collector

Heavy rocketchat.users collection updates

Relevant logs:

Begin with the presence broadcast behavior and the heavy rocketchat.users updates described in the report; reproduce the reconnect surge in the listed 3,000-user, 33-instance Docker Compose setup. Done means reconnecting users no longer makes Rocket.Chat unavailable while presence remains accurate; use the reported CPU, RAM, garbage collector, and users-collection symptoms to verify.