This is the first time I've tried to use the arpeggiator, being mainly an audio guy.
I want to turn it on and off a number of places mid track, but I've found that the Enable/Disable automation only works consistently when the MIDI track is selected. When it fails, it's not consistent, but frequently fails to either turn on or off.
The work around would be to bounce the track to audio, I suppose.
Must be a bug, I can't see this as being normal expected behavior.