Me, doing QA

PM: "stop submitting bug reports about screen size, we're only supporting one resolution for now"

Me: *What do you mean you're only supporting one resolution it's a website and it breaks on screens <1400 px tall*
*Sigh*
"okay, what resolution?"

PM: "No one knows"

Me: *dies*

This is, sadly, very relatable.

That's exactly why you report it, as you have been doing. It's called CYA. The bug comes in and they can determine if it's actionable or not. As a QA engineer it's not your job to determine what is actionable, just what does and doesn't work. If they deem it not actionable, it becomes documented as a non-issue instead of some mysterious unwritten requirement in the application that then turns into finger pointing down the road.

I don't have a lot of confidence in your PM from what you've described. =/