When viewing this site (main or meta) in my main browser, there is an empty 'SPONSORED BY' label just under the top bar:
When viewing in another browser, the AWS logo is showing correctly:
Catija's hunch was right: the logo is blocked by uBlock, since it's a link to ad.doubleclick.net.
I would have checked that, if I hadn't seen the AWS logo on Cross Validated a few weeks ago. But right now it's not showing there either. I guess the ad link is important enough (in terms of revenue, or just part of the sponsorship contract) to keep it, so I'm no longer considering it a bug.