Hi Otoy
I've been instructed by Paul (Face_Off) to bring this issue to your attention.
THis has been the Modo thread;
https://render.otoy.com/forum/viewtopic.php?f=34&t=61192
I've exported the scene to the stand alone and I get the exact same issues.
In short, the shadow catcher darkens when I have the meshes with emissions tagged to them. Turn those off and the shadow catcher works as intended.
Thanks in advance.
Josh