Hello Alex,
I suspect that particular organ is over-filling your RAM on that particular macOS version (which may need more RAM for itself than previous macOS versions). macOS is particularly sensitive to a shortage of RAM, often resulting in no audio.
As a test, just to see whether that's the problem, try loading the organ via 'Organ | Load organ, adjusting rank audio memory options', highlight all ranks, and change their channel format to mono (which will use about half as much memory).
If that eliminates it (as I think it will), then it indicates that you were indeed over-filling the RAM when they were loaded in stereo, so when setting them back to stereo you'll need to adjust some rank options to save enough memory to keep macOS Big Sur happy.