When I started hosting a game today(we were on about day 225), we had a couple of thermal stones that were no longer able to get cold. This had never been an issue before. We are storing them directly in snow chester, and they are only in the neutral state. I had no issue warming them up, just getting them cold. I tried a full sized endothermic fire in the dead of winter as well, which also did not work. I then tried crafting 2 brand new stones, which also could not get cold. Lastly, I tried shutting the game down and reopening the map, which sadly also did not solve the issue.
Unknown. Thermal stones had previously been stored in snow chester for about 100 or so days previously. Perhaps the length of the current game, being around day 225?