I tried the book in KFX format on a Kindle Oasis and the gray rectangle was present as you showed. I also sideloaded it to the Kindle for Android app and Kindle for PC. Both of those showed it correctly, without the gray rectangle. So it appears to be a problem specific to e-ink Kindles.
As far as I can tell, in KFX format on an e-ink Kindle an <hr> is rendered with an implicit gray background color. It can be made darker, but not lighter.
Changing the EPUB so that the image is rendered using a <div> instead of <hr> causes it to work properly even on the Oasis. I don't have a work-around other than making this change.
(Thanks for the learning experience.)