I have been having the same issue. I think it is fair enough if you get hypothermia in the rain if you are inadequately clothed. However at the moment there seems to be a bug with the system where a fully clothed player is always getting cold regardless of the weather.

With my pre patch character I became hypothermic and died not long after putting on new boots at the NW airfield. So did the other person I was playing with. My new character had the same issue until I swapped boots.

!!!!!!!!!!!Figured it out!!!!!! (For me at least)
Just been playing around. I have found that swapping to some different boots fixed the problem for me. I went from freezing to just cold within 5 seconds when I swapped from pristine hiking boots to pristine working Boots.

https://www.youtube.com/watch?v=TLlVh6AxlnM&feature=youtu.be

I'm currently fully clothed and dry and can do nothing to stop myself cooling off. I have gone from being fine to freezing while fully clothed and dry as I showed in the screenshot earlier. I can't see my breath so it surely isn't too cold. It appears sunny and no matter what I end up hypothermic and dead. http://steamcommunity.com/sharedfiles/filedetails/?id=318350836

Lucky you not having this issue. Doesn't mean the rest of us aren't.
Surely one of my items either my puffer jacket or riders jacket would of kept me warm. I never had this problem until last nights changes. Previously managing a player well would keep me warm. Especially when it is perfectly sunny and a player is fully kitted out. However now no matter what I just end up dying of hypothermia.This screenshot was after about 30 minutes.

http://steamcommunity.com/sharedfiles/filedetails/?id=318350836

I had the same issue with winter hunter pants and other worn dry clothes. Calm and sunny while I'm Hydrated and Energized. Died within 10 minutes of starting to get cold. Tried picking up new clothes and that didn't help either.

I have been playing since then without becoming freezing (Or cold even) with all the same gear. I was only running on the spot for less than a minute. I swap the boots then start running and hit tab and instantly I am just cold.

The day before I picked up some boots while fully kitted out with military gear and withing about 15 minutes I was dead from hypotermia. For me at least it has been an issue with the boot.

Figured it out (For me at least).
Just been playing around. I have found that swapping to some different boots fixed the problem for me. I went from freezing to just cold within 5 seconds when I swapped from pristine hiking boots to pristine working Boots.

https://www.youtube.com/watch?v=TLlVh6AxlnM&feature=youtu.be

@Nothingstrue

Normally some combination of this kit would keep me warm. As of today whatever I do I end up with hypothermia and dead. I have seen people saying that they can only warm up on some servers even in the same weather conditions. I think there has been a bug introduced in the latest update as I have never had this issue before today

http://steamcommunity.com/sharedfiles/filedetails/?id=318350836