Here's a simplified explanation:
This is a problem with Dark Mode and emails/web pages that use "custom" background colors set to white, and emails/web pages with backgrounds with no custom color — which generally appear white but are actually transparent. These are the emails you're referring to. They have a transparent background, so the "dark" background obscures the dark type.
Most email apps use a custom white background that Apple's Mail app recognizes when in Dark Mode, but some do not. I believe Gmail is one that doesn't use custom backgrounds, so any email received from them in Mail while in Dark Mode will appear to have black text against the dark (black) background.
has learned already, this isn't an Apple Silicon vs. Intel chip issue... it's just the way Mail respects the way the email was coded, rather than forcing some implementation that only works half the time.