The following code demonstrates the issue:

import numpy as np
from multiprocessing import Pipe
p1, p2 = Pipe()
arr = np.zeros((3, 5, 6), dtype=np.uint8)
p2.send_bytes(arr)
pm = p1.recv_bytes()
print(pm)

Only 3 bytes are transmitted for this array which obviously consists of more than 3 bytes.