This is not a new "glitch" (and frankly, I'm not entirely sure it IS a glitch, based on my exchanges with the KDP service reps). A few weeks back, there WAS a glitch where random books were showing up as "unavailable" in the US store to US customers. As far as I know, that issue has now been rectified--one of my titles was affected but went back on sale in a couple days--but watching the availability behavior of books over the past couple weeks, I'm becoming more and more convinced that that original glitch was an accident caused by intentional changes to regional book availability that Amazon is currently in the process of making.
When I released my latest book on 11/2, I got emails from multiple non-US readers who frequently purchase books in the US store, telling me that my new release was showing as unavailable to them. Sure enough, when using a proxy server from their countries, the book does indeed show up as "unavailable."
Looking further, I found that if I go to my KDP dashboard, hover over the LIVE indicator, and click through to "See title availability details," my new release (and, in fact, all my books now--because it's SPREADING) are showing up as "Limited Availability" on the US and UK sites. Clicking the dropdown shows that there are numerous countries whose residents cannot purchase my books in those stores, and that includes people from Australia and New Zealand.
Interestingly, the availability of my books in the US store to AU/NZ residents keeps alternating. They were available again the other day, then suddenly switched back to unavailable again today. I have no clue what that's all about, or why the regional unavailability doesn't ever apply to every book in the store, just a seemingly random assortment.
Anyway, after my readers started emailing me, I emailed KDP, and after a bit of back and forth, I got this interesting runaround answer:
Limited Availability :
As you can see under "Limited Availability "there are few countries where your eBook is not available for sale.
Kindles and Kindle content are currently unavailable in those countries because of country/region specific restrictions, your title is not available on those locations.
We value our international customers and hope to make Kindles and related content available in more locations in the future.
I found that last line particularly interesting, because I know for a fact my books WERE available to people in those regions as recently as a few weeks ago, and I also know for a fact that many books still ARE available to residents from those regions in the US store. It's all very...odd.
Now, I'm not going to claim I have any idea what Amazon is plotting behind the scenes, but I'm struggling to figure out how this unavailability issue, which has now persisted for weeks, can actually be some random problem introduced by a glitch in the system. The longer it goes on, and the more answers like the above I received from KDP, the more I'm convinced this is in fact the result of planned, large-scale regional availability changes on Amazon's part (changes that, like all major Amazon updates, have resulted in glitchy and inconsistent behavior in the stores).
That's my two cents, anyway. I may be totally off base, but that's the best I can come up with.