I'm on a Pixel 3. It's just solid red - no faint logo.
It might be included only in the Android Q update.
Googling around shows that it might be restricted to custom apps only?
"There is one way to bypass this issue, and that is to explicitly tell Chrome to override the default provided system colour profile and just tell it the display is Display P3. This enables colour management in the app, and short of writing a custom application, is the only way to get wide colour gamut content to work on the Pixel 3."
https://www.anandtech.com/show/13474/the-google-pixel-3-revi...
Take a screenshot of the full-screen image. Do you see the logo flash?
Zoom in to the image. Do you see the logo appear, but only near the edges of the display?
Computers!
On my Pixel 3 XL, the behaviors I described occur, and also the logo is invisible unless the overlay is shown (in which case it's drawn only near the edges, unless you're taking a scree...oh.
I get it.
Image math is getting done internally in a wide gamut. Including alpha blending. But it's converted to sRGB before being displayed.
The things I described all, I suspect, modify the image- mostly by just dimming it, and the resulting dimmed wide color values map to different sRGB colors (where the undimmed colors all map to the same shade of red.)
as to what's actually happening: not a clue! just pointing out possibilities :)