Another note - the original problematic code looks like this:

def non_decreasing(L):
    return all(x<=y for x, y in zip(L, L[1:]))

Changing it to:

def non_decreasing(L):
    def f(L):
        return [x<=y for x, y in zip(L, L[1:])]
    return all(f(L))    

also worked around the bug