You are totally right: I just found this independently a couple days ago, and the fix is to simply flip the conditional operator:
https://github.com/OpenSprinkler/OpenSprinkler-Weather/commit/bf538ab0854a85ce6461772be1ef4e1d5a16f0bf
because NaN compared to anything would return false, the fixed code will assign 0 to the precipitation data if it’s NaN. The code has already been deployed. Let me know if you’ve observed any changes on the watering percentage on your side.