Filmstrip artifacts and skewing start render

On occasion, in our private instance, I’ve seen instances where the mask or however WPT prevents scrollbar and loading message from skewing the start render metric be slightly off.

In the past an upgrade fixed that when it was a mobile issue, but we’re again seeing this on desktop tests, but not all agents, and not all tests.

Are these issues configuration of the agents/browser, a WPT code issue, or WPT versioning issue or a library issue?

I’m wondering how to fix it/what causes it so I can tell my IT team how to address it.

Any help would be appreciated, thanks!
A coworker also asked the question:

Is the loading msg masked until content appears, and the mask is removed, or is the status/scrollbar always supposed to be masked.