Five days after I first noticed it, the ability to upload a social preview for a repository has been fixed.
So now I can finally update the preview for Rogallo again.
From what I can see it took a couple or so threads in the community forums to get this issue noticed, and at least one person in those threads said how they'd initially raised a ticket to report it as a bug and had been brushed off and told to go to the community, as if it was something they were doing wrong rather than reporting a genuine issue.
This has been my experience in the past at least once. I forget the actual issue now, but I remember stumbling into a problem, being a good user and reporting the bug, along with details on how to reproduce, etc., and having the ticket closed and being brushed off with some vague and unhelpful reply. My recollection is that I then found a thread or two where others were talking about the problem. Eventually it was acknowledged and fixed.
It's hard not to feel like such an approach is a deliberate policy. It feels like their front-line support is now a shield that deflects problems to a place where community triage takes place.