There is a bug here.
You can check your .conf file permissions.
The root cause of crashes or other unexpected behavior will be in the error logs. The default path of the MySQL log file is /var/log/mysql/error.log on Ubuntu.
Also for debugging check this out.
MultiSubnetFailover can be used only for connecting to SQL Server part of the client connection string.
For MySQL, if you are using .NET, then you can use multiple hosts in connection string or you can even use single DNS that provide DNS Service (SRV) records to provide failover, load balancing, and replication services.
Sounds like a hacker.
Based on your graphs, I suggest that max_connections = 4000 is unreasonably high. It looks like 20 might be reasonable.
You might want to set max_user_connections to some reasonable value (instead of 0 for "unlimited").