select is documented specifically to give the first condition priority if multiple are true.
piecewise does not document what happens, but does the opposite:

>>> import numpy as np
>>> x = np.arange(5)

>>> np.piecewise(x, [x > 1, x > 3], [1, 3])
array([0, 0, 1, 1, 3])

>>> np.select([x > 1, x > 3], [1, 3])
array([0, 0, 1, 1, 1])

We should either:

• Document the fact that piecewise does things in reverse order
• Swap the order of piecewise, and document the new order
• Emit a deprecationwarning and eventually a ValueError if multiple conditions are ever true, so that the point becomes moot.