[Bug]: alpha array-type not working with RGB image in imshow() #26092

Bug summary

Hi,
Whereas alpha = constant works with RGB image, this is not the case when working with an array-type for alpha.
(In my real case, I can only pass standard imshow parameters like alpha to the function of the library I use, and not a RGBA array).
Patrick

Code for reproduction

import numpy as np
import matplotlib.pyplot as plt
from skimage.color import gray2rgb

arr = np.random.random((10, 10))
arr_rgb = gray2rgb(arr)

alpha = np.ones_like(arr)
alpha[:5] = 0.2

plt.figure()
plt.tight_layout()
plt.subplot(121)
plt.title("Expected outcome")
plt.imshow(arr, alpha=alpha, cmap='gray')
plt.subplot(122)
plt.title("Actual outcome")
plt.imshow(arr_rgb, alpha=alpha)
plt.show()

Actual outcome

Expected outcome

Additional information

No response

Operating system

Windows

Matplotlib Version

3.7.1

Matplotlib Backend

TkAgg

Python version

Python 3.10.11

Jupyter version

No response

Installation

pip