Can not use transparency in px.imshow

I am using plotly 4.12.0.
The px.imshow function has this example in the docs:

import plotly.express as px
import numpy as np
img_rgb = np.array([[[255, 0, 0], [0, 255, 0], [0, 0, 255]],
                    [[0, 255, 0], [0, 0, 255], [255, 0, 0]]
                   ], dtype=np.uint8)
fig = px.imshow(img_rgb)
fig.show()

I have tried to add transparency to the plot with the following code:

import plotly.express as px
import numpy as np
img_rgb = np.array([[[255, 0, 0, .5], [0, 255, 0, .5], [0, 0, 255, .5]],
                    [[0, 255, 0, .5], [0, 0, 255, .5], [255, 0, 0, .5]]
                   ])
fig = px.imshow(img_rgb)
fig.show()

As suggested by the docs, passing a (M, N, 4), array would include transparency, but this resulted in an empty figure:

Using go.Image directly worked:

fig = go.Figure()
fig.add_traces(go.Image(z=img_rgb, colormodel='rgba'))

Reproduce the empty figure with the reported px.imshow example using a (M, N, 4) array, then compare it with the working go.Image example using colormodel='rgba'. Trace how px.imshow handles four-channel image input; done means px.imshow renders the image with transparency as documented.