I have no solution. But as your K9 seems to use a different theme for message view (light) than for composing (dark) it might be related to it. Could you try to use a single theme, at least as a test?
Does this happen with all mail or only selective such as GitHub?
It might be an issue with improper HTML and plain alternative. A while back I had similar issue with mails from one sender (a newsletter). After changing my preference to plaintext only (i.e., disabling HTML), the newsletters were displayed properly.
However, this obviously was sender-side setting in my scenario. I do not know if it applies to your case as well.
A different idea I just had: Have you tried emptying K-9’s cache and force re-downloading the mails?