I'm reporting this as a "bug" because I believe it to be an unintended and undesired consequence.
The current winter was postponed twice. As is my custom, I had placed my mound beds in late fall, and planted herbs in them to make sure they wouldn't go away. This morning I got "you cannot plant in frozen ground" in an area obviously covered by a mound bed.
Sure enough, the mound bed disappeared when I harvested its herbs. I replaced it, but don't have enough mulch on hand to replace all my mound beds, having planned based on a normal winter. And crops that went to 0/0 don't go back to where they were just because there's a new mound bed.
I'll update the mound bed wiki page to say that mound beds stop working for climate control _with no visible indication of this_ at the point when they would disappear if you harvested their crop.
But please, next time you change the calendar this way, please set all mound beds back to a brand new status. (Or just subtract the appropriate amount of time from their clocks.)