I've just got to the outside section of tomb raider before the climb to the radio mast. Because of the lighter background tones I have realised that there is the ghost of the title image in the screen at all times, the title of the game and Lara with her bow underneath.
It's really faint when the camera is stationary, but really noticeable when you move the view.
I've tried unplugging the Chromecast ultra and plugging it in again but it doesn't help.
My connection is just over 50 mbs and as long as no one else in my house streams video, generally solid for stadia.
Any help would be appreciated.
This is a general problem that some people have been reporting recently, I haven't seen it happen yet but all I play is Destiny on Chromecast on an "Excellent" connection so I'm not sure if screen type or stream quality has anything to do with it or not but I don't think there is anything you're doing wrong.
I'm pretty sure they're aware of it so it's a wait for a fix type situation, I believe.