I think I did literally this exact thing the other day - when the garbage matte was connected to the camera input (via an “in” node), the camera feed would disappear. I was using Holdout and changed it to Garbage Mask and it worked fine – despite that the video from UE4 that I was watching clearly showed using it as a Holdout. I figured that either it had changed in UE5 or that there was for some strange reason an inversion somewhere, but just didn’t have time to deep dive.