Randomize Header Images not random

Using WP 5.5.3, with Twenty Eleven theme. I have uploaded 24 header images and selected Customize -> Header -> Randomizing Uploaded Headers.

The problem: the site displays some of those images, but definitely does not give a uniform distribution. Certain images (not always the same ones) appear three or four times, while others are not shown.

Specifics: I refreshed the home page 35 times, and got the distribution of images below. 19 images appeared (some multiple times), 5 were not shown.

4 bent branch
4 tennis court fence
2 compass hill view
2 foggy top to post hill
2 Holts ledge (little field in center)
2 mirrored sunset
2 orange sky
2 second mirrored sunset
2 house with Christmas lights
2 sun shining on frozen pond
2 sunset with big pine in center
2 trail to franklin hill
1 christmas trees on common
1 foliage on papoose trail
1 hoarfrost
1 holts ledge horizon
1 ice flowers
1 lake beach
1 sunset on snowy field

My question: This doesn’t seem random. Is this a bug? How would I submit a bug report? Many thanks!

PS I hope you enjoy the images!

The page I need help with: [log in to see the link]

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