No change was made intentionally, but probably this was caused by a Discourse server update? Can you send the html source of the “before” email and the “after” email?
FWIW my emails look fine before and after, but I notice that I have a white background in my email client, and you have a dark background - my guess is that somehow the background color of the code blocks is being explicitly set to white now, which may conflict with dark mode on email clients? Is the code present but white?
Your guesses sound plausible to me; yes, the code is present, just rendered in the (more or less) same color as the background, so invisible. If you’re right about the hard coding of the bg color, i guess it would be a sort of thinko/bug to not also hardcode the font color … though from my pov, they could have just ain’t broke, don’t fix it, it looked fine before.