I have your same problem. I have around 10 or so TBLeague ladies and two of them suffered the same fate. The first one was the original S02 body (the really tan one, not the second lighter version) which "melted" in a worse manner and I had to throw it away. The second one is what I believe an S10D body. Interestingly, I have a couple of S10Ds but only one of them melted that way. I just took pictures of it because I still haven't thrown it away because I was thinking of using it for a "Lady Terminator" character if she melts and loses more flesh. Take note of the yellowish, probably oil, substance on the tissue paper that I used to wrap it in. There used to be more of those yellow stains whenever I changed the tissue paper wrapping before. Also, the problem of the flesh being like jelly or melted are concentrated on the arms and the legs. The main body itself still feels firm and perfect. I noticed this similar melty jelly effect on the smaller original 1/12 TBLeague lady as well.
As for conditions, it's very hot here in my part of the USA, to the point that metal door handles can get hot, and these ladies aren't touching anything else. For posing, they were just standing normally, nothing special. It's just those two which inexplicably melted while the other ladies, even older models from 2014 and around that year, are still in great condition here and have never melted. If this kind of thing can happen in a cool place like yours, I imagine we unfortunately received "bad" batches. I have other S10Ds that never melted.
Take note how even the tissue paper made marks on the flesh.
I have no idea how to explain this... it's like her flesh became a suit that you could pull up and down.
The area near the elbow just split open in a very clean manner.
She has very gelatinous thighs.
Her skin is also peeling off.
The tissue paper I wrapped her in has yellow oily stains, there used to be more in the past.
The fleshy area of her arms are really slushy!