I experienced this as a total switch - before, the site always kept me logged in for several days, afterwards it always makes me log in again after a day or less away. I always selected "keep me logged in" but that is no longer offered, and the user name box now blocks autofill prompts by the browser. The blocking I have for cookies is all waived for FMP's site. I'm not sure if the forced log-out is timed at 24 hours, or sometime overnight, or a bit of both.I've just looked at the user profile, and can't see any setting that disables staying logged on - the only recently new option is 2FA (which is turned off).
As to when it changed - I can't now remember (and I'm no good at that sort of memory recall anyway). As I remember it, the change coincided with some other changes to the site's appearance, but was before the new interface for the Newspaper Archive came in.
Incidentally (or not) I just logged out so I could see the login page again - and my login failed with an "oops" message. When I returned a few minutes later I saw logged-out home page, but as soon as I selected "log in " I found I already was. That suggests to me that something in being tinkered with.