I have not bothered to actually check

Then I'm certainly not going to. I've had this problem on a quite inconsistent basis for a very long time. I now have my browser configured (well, IE, my other browsers either don't make it easy to do this configuration or have other problems that make me use them only for things other than PM) to *never* invalidate pages that are in its cache. It still does invalidate pages but only very rarely (probably because of some header, but I have not bothered to actually check since it happens so rarely for me now).

I did quite a bit of digging to try to figure out why IE and my phone's browser were so aggrevatingly fickle about when they'd insist on refreshing instead of just using what was clearly still in their cache. I never came up with an explanation (sometimes they'd be happy to show me a copy of a page I had fetch many days ago and sometimes they'd refresh when I was going "back" to a page that I had loaded just a few minutes or even seconds ago).

I'm working on merging patches to Everything/HTML.pm so we can start sending reasonable cache control headers in hopes of making this less of a problem, among other improvements (thanks, Corion).

Your problem probably has nothing to do with any changes at PM (I doubt we've made any, but I haven't bothered to check my recollection). You might want to bother to check now. Maybe you'll figure out something helpful.