patternrec_dtw with rolling is not working properly

I have an issue when applying patternrec_dtw to my dataset. To test it, I have set max_distance=0, hence only the same day as the pattern should be flagged as pattern. The function isPattern(chunk) correctly returns FALSE for all the chunks - but then all the days get flagged as pattern. Whats the reason for this?

"""

ref = data[ref_field]
ref = ref.to_numpy()
ref_var = ref.var()


def func(a, b):
    return np.linalg.norm(a - b)

def isPattern(chunk):
    dist, *_ = dtw.dtw(chunk, ref, func)
    if normalize:
        dist /= ref_var
    return dist < max_distance

dat = data[field]
sz = len(ref)
mask = customRoller(dat, window=sz, min_periods=sz).apply(isPattern, raw=True)

flagger = flagger.setFlags(field, loc=mask, **kwargs)
return data, flagger

"""