I notice something similar on SFOS 3.3 on my Jolla 1. Sometimes, the content seems to dim or fade away with arbitrary and completely random targets (I mean opacity values). The content may become completely unreadable or hardly readable, depending on its final opacity. When this occurs here, I tried to trigger redraw events by switching to events display or making it show its cover, but to no avail: whenever I returned to the Messaging app, content would stay dimmed.
I have to try rotating the screen!