I'm not sure how the original problem started - it might've been from a mod, but the problem persists even after disabling all mods. I opened up Debug mode to try and track it down but I'm not a great programmer, so this is what I found:
At some point (it would seem) an item of clothing had picked up, in addition to its usual attributes, an additional "DBG: wetness:" (note the lower case 'w' in the label) attribute - this gets listed in text form below the bars for the stats (as shown). If this item is worn, it corrupts the players stats so temperature/wetness just read "NaN" and the character is no longer affected by the climate (even if you unequip everything).
This can be fixed by removing all items of clothing, opening debug mode, enabling then disabling God Mode.
Now if you create a new item, its tooltip just shows the normal bars and item IDs. If you wear the new item and nothing else, it's fine.
However, if you put on one of these 'bugged' items, then the character breaks and all items you are wearing become bugged.
Not sure if I'm just tired and missing something blindly obvious that I've done, but if not, I hope this helps. :]
