I upgraded the Mac mini last night to Big Sur and FCP 10.5 for testing, and am also getting this behavior on some plugins.
So far it's been an issue for me on everything in the C2 Chromatic package, Advanced Reflection in the C2 Gadgets.
I did have some luck with C2 Lock & load and was able to apply the stabilizer without issue. The plugins I've checked so far do appear to preview correctly when scrubbing, but I get the beach ball after applying the effect to a clip and have to force-quit FCP. It seems like they're not applying to the video stack properly.
Hopefully CoreMelt will get this sorted fairly quickly, but for now I'd keep your production machines on Catalina.