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.

I believe you want to modify your sending of bytes to read:
p2.send_bytes(arr.tobytes())

The docs on send_bytes explains that it expects a bytes-like object.  NumPy arrays do not qualify by themselves but they can be readily converted.

To reconstruct that array from the 90 bytes received into the variable pm:
np.frombuffer(pm, dtype=np.int8).reshape((3, 5, 6))

Marking as closed after providing an example of how to send NumPy arrays as bytes with the send_bytes() function.