I checked for light leakage as I was shifting my 72mm lens 18mm left and right in portrait position and noticed the light anomalies in the shifted images. I have a Lupine LED flashlight that has a beam brighter than the sun and shifted left and right with the lens open and couldn't detect any leaks when shining the light all over the bellows.Sorry to hear that Victor. Can I share my experience with you about my experience with GFX and Actus. like you I have using this system for awhile, I have even modified the bellow to a custom made lamb skin below, very narrow and almost no distance to keep my flange as close as possible as GFX flange is bit on the long side. One thing let to another, one day, I discover a long stretch of fog/red patch on my images while using GFX to do some stitching works. I thought it was my lens that is the problem. But ended up, it's camera handle pushing the bellow too much until a small gaps has appear at the magnetic seam.... light leaks.... And then another time... the same problem occurred, and I have check the connection of the bellow, everything seems fine.... I thought, this must be the lens again! only later to find that, again, it's the bellow.... a screw has gone missing at the ring, and it make the bellow slight loose on the metal ring part....
All I want to say is, check your bellow, cause it might have been push out of alignment by the camera, particularly this GFX100 as the size is just way too over and generate light leaks...
By the way, Cambo is coming out with a new attachment and shim for GFX100 to work on Cambo Actus.... so you can do a normal landscape shoot, except that it add 12.5mm to the flange... ouch....
Victor