When you do:

    FINUB = np.empty(len(close))
    FINLB = np.empty(len(close))

you're creating two *uninitialised* arrays of values. (See the NumPy documentation at https://numpy.org/doc/stable/reference/generated/numpy.empty.html.)

When you then do 

    FINUB[i] = UB[i] if UB[i] < FINUB[i-1] \
            	and close[i-1] > FINUB[i] else FINUB[i-1]

on the first iteration of the loop (i = 1), you make use of the (undefined) value in FINUB[0] to compute FINUB[1].

In other words, this is a bug in your code, rather than in Python or NumPy.