Undo log purge thread stops in replication slave

Currently, it operates with the configuration of mariaDB (master / slave) + maxscale.

About once a week, the undo log purge thread stops on the slave side,
There is a phenomenon that ibdata1 is enlarged.
(the master side is normal)

Contents of SHOW ENGINE INNODB STATUS when the phenomenon occurs:
[Attached file]

History list length becomes large, and "1 read views open inside InnoDB" occurs.
It seems that the purge thread has stopped.

This phenomenon can be solved by stopping replication with "stop slave".

Where is the cause?
There may be a problem in the operation between master and slave.