MariaDB(10.4) is sometimes very slow

I have a super production DB server as above environment.
DB servers consist of 1 master DB server, 2 replication servers.
And there are 3 WAS server and 1 RP server.

Generally, DB server used 10~30% cpu rate.(active db sessions are 30~40)

But DB server is sometimes very slow.( 1~3 times a week)
So actvive db session increase to 3000(max), and cpu rate up to 30%~60%.
And our service have been stopped.
This problem occur at non-peak time.
This is about master DB server.

I've googling for a month, but I didn't get any reason and solution unfortunately.
I doubt innodb_log_buffer_size, but I'm not sure.

Currently, our only solution is that WAS servers httpd deamon stop, and restart after active DB session is 0 from WAS servers.

I attach mariadb variables and innodb status of problem's initial time and innodb status of normal.

What can I check?
I hope this nightmare is ended ASAP.

Thanks in advance.