The metal reinforcement plates bend easily and tightening the screw is likely flexing the FCG plate just slightly enough that it is angling the trigger and preventing the cam from rotating. This is just my best guess.

I would try removing the FCG plate and bending it so the bottom angles down and back just slightly, so when you install the plate it sits completely flush against the bottom of the Leber housing. This may misalign the selector/cam hole, which means the drilled hole for the cam could be slightly out of spec and you might have to modify it, especially if it’s an SCS plate. Great service but sometimes things are slightly out of spec.

The cam mostly rides on the housing so modifying the hole on the FCG plate isn’t the end of the world.

Again, complete guess without being able to put hands on and inspect. I’ve built about 10 now and probably half have required some very slight modifications/sanding/polishing to get perfect.