FWIW: I had similar, but much less drastic experience with the low HTB fees at TDA. I think they may have been something like 2% or less when I began using them which looked very good to me, and one day observed 13% with like a 4% increase since prior day (I had ceased monitoring the daily rates as well). This prompted my transition to alternative shorting, as I was not in control of the costs.
I didn't think you could short HTB stocks with TDA unless you're enrolled
in the "HTB Program"