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All I can say is same here. This makes working with a multiply mode brush very complicated. I was hoping there was a workaround. Guess not?
The only thing I can think of of is inverted colors and add, and then inverting everything back again in After Effects haha

Here is what WORKED for me: I was encountering the same issue on my Mac. (Catalina) On my other Mac (Big Sur) everything works fine. My SAVE window would freeze. If this is also your problem, you can rescue your project by doing this -> Open your ACTIVITY MONITOR TV PAINT will (probably) be first in...