After the last update of
EP-1
1.0.5 → 1.0.8
ARP-2600
1.0.6 → 1.0.9
VOX Super Continental
1.0.5 → 1.0.8
the on/off parameter control no longer works properly with these plugins.
If you assign the FX1 enable parameter to a button, for example, you can switch the effect on or off once. After that it no longer works.
With the previous versions it still worked correctly.
I have reported it to Korg. Maybe someone else can do it so that they understand the priority.
I noticed the same thing today with EP-1, and was about to post a question here whether I was doing something wrong.
Did you hear back from Korg?
No. I never get a feedback from Korg. 
Too bad. I raised a bug report to Korg, too. Let’s hope they come up with a fix.
Good news! With the new versions the bug is fixed:
EP-1
1.0.5 → 1.1.2 
VOX Super Continental
1.0.5 → 1.1.2
But there is still no update for the ARP-2600… 
EDIT: Today ARP-2600 V 1.1.4 is released - the bug is now also fixed