This appears to be a bug that has been around for some time and/or a leftover from the past with a misleading status for an EVENT on a master node.
I can't think of a case were changing an EVENT to DISABLE ON SLAVE would be useful, since by default CREATE EVENT sets the status on the slaves to SLAVESIDE_DISABLED.
"DISABLE ON SLAVE" used with CREATE or ALTER EVENT should either:
A) NOT change the EVENT status on the master node to SLAVESIDE_DISABLED and keeping current ENABLED or DISABLED status on master. Or act like the DISABLE option and keep the event status on the slaves to SLAVESIDE_DISABLED.
B) Remove the DISABLE ON SLAVE option altogether from CREATE or ALTER EVENT. DISABLE can be used instead. Having the status SLAVESIDE_DISABLED on an EVENT on the master node is misleading.