Archetype Petrucci Doubler problem

When the Doubler is active, you get this beautiful stereo effect, but this only works when the sound has the timing difference between the left and right speaker, of course. When the monitoring comes from a mono source, it turns into a very hollow, very heavy chorus. This undesirable chorus is not what the sound of this amp is about.
I understand that the effect of the timing difference changes when played over a mono monitor, but it shouldnt change into an unwanted effect. If I want a chorus, I’ll turn on the chorus, I dont want a chorus when I dont ask for it, of course.
What can I do to avoid this?

How are you summing to mono?