Since updating to 15 I’ve had issues with this kind of text display. This, for instance, is in Messages. Messages itself is set to follow the system theme, and the system theme is set to dark mode. If I set it to light mode the text displays as expected. Trying to manually set the theme in Messages doesn’t fix it. Nothing special going on with the phone itself - it’s a Pixel 6 Pro just running regular Android. Thought I’d post in case anyone has seen something similar.
Seems like the bug where random things become light mode, on Android 15. I think you can toggle off and on dark mode and it’ll be fixed, or change your colour theme to something else and back.