Just finished answering a question when I saw a notification in my inbox. Upon opening it, all of my notifications were gone. Checked with browsers I don't usually use and ones that have an incognito mode and they're all empty. Visiting other SE sites (other than Meta) shows the same issue, but I can see my notifications here. I've seen a few other topics with this question, however I haven't been away and didn't see anything about server maintenance. Is it just me?
July 18th update: happens again for at least five hours now, maybe more. The inbox in Stack Exchange™ MultiCollider SuperDropdown™ is empty, while in the inbox tab of my network profile it's fine and showing all items. Also worth to mention that the raw request to http://meta.stackoverflow.com/inbox/genuwine returns