On edge, inspecting cookies, deleting one by one i isolated it- its under 'br.' (pinterest opens with the br before in my region), under service worker, called br. With some trial and error i figured out a cookie is saved on first load with the login thats causing it. I have used 5 other chromium based browsers recently, that doesnt happen in any of them.

I think i recall that not happening on edge when i first tried it, so i suspect an update that brought 'integration' with pinterest is when the problem was created- btw, this particular 'mobile view' (entirely on users end, url doesnt change to mobile pinterest) is exactly like the installable 'web app' (installing pinterest site as app via edge, or installing pinterest in the microsoft store). The issue: soon after login, after any reload of front page or second visit, pinterests site becomes a specific kind of mobile view (and pretty much unusable to me). Had this issue in 2021, came back to edge its still here- posted on the community with no solutions

Microsoft edge mobile