I've uploaded a perfectly normal PNG as the cover image for an unpublished project of mine, but when I hit "View Page" it only displays my uploaded screenshots - no cover! Does anyone have any experience with this? I saw a post from three years ago with the same issue, where they blamed it on their Firefox browser, but I'm on Chrome.

I'm not sure what you're asking. Game pages normally don't display the cover, just screenshots (unless you don't have any).

The cover image in Edit Page is the thumbnail you see in your dashboard. Do you mean the banner image you upload in Edit Theme?

i just remembered, the cover will show if there are no screenshots but otherwise just the screenshots. At least that seems to be the case in my game pages.