I think it's a problem on Google's end.
I keep a spreadsheet monitoring my GWT sitemap stats on a site and I have seen the exact same behavior. The number of pages indexed on Jan 20 was different from the number when I checked Jan 10, as it nearly always was every few days I checked for the last three and a half years, but since Jan 20, the number has not changed even though the number of pages submitted has increased and I have even added another sitemap.
Like you, all other indicators seem normal, and the site is a fairly dynamic, large, well-indexed site (well over 100K pages) that, until the last three months, had previously never had the same number of Indexed pages for more than a week.
I've been wondering if it was a bug but I couldn't find anyone else on the Internet talking about it and as I've often seen different aspects of GWT be temporarily slow or outdated, I kept expecting it to fix itself and haven't looked into it further. I'm guessing it doesn't effect everyone or we would have heard about it elsewhere by now (though anyone could confirm this if your Indexed numbers have changed since mid-January), but I think you've provided evidence that it's more than just me.