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.