We have not had this report until now. Ideally their spout output should be synched on the frequency of the projectors MadMapper sends video to. To make things simple, it's recommanded to have all screens on the same frequency, even if MadMapper handles that the screen frequency where the UI is can be lower than the output ones (it will then skip renderings on the preview windows).
I suppose all your screens are at 60 Hz ? Then Magic should ideally run at 60 FPS. If it runs at unlimited FPS, it will consume all GPU resources and will not let enough for MadMapper to render on the outputs.
If you use a material in MadMapper instead of the spout input, everything's ok ?