Yes it’s a system thing (read my answer in the link provided by @Justin_Ross).
My guess: you are simply using Nomad differently than the other apps.
I got Procreate, and I can confirm it’s the same behaviour (unless you fiddle with their Settings in the iPad Settings App).
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Undo have nothing to do with Palm rejection.
Palm rejection simply means: the iPad will not send some inputs if it detects the input area is too big.
The undo and redo can be improved/tweaked but that’s another request.