HF stands for "Hot Fix". It represents an emergency patch released by the game developer in order to fix a showstopper issue within a normal patch. A Hot Fix will not increment the version number of the game but may cause enough code change to need to update the trainer.
From what i noticed it happens only when activating F11 easy win cards without being in the gwent game. Deactivate that option and all should go back to normal.
Yes, it worked fine for me when I played a few months ago. I played the GOG version with all DLC's and friends played with it on Steam.
Sometimes I lose health, sometimes I don't. As it says in the Readme, if you level up or after a cutscene you might need to press the option again to go back to a million.
I just fought a Vampire and let him bang on me and I barely lost any health at all. Next fight I lost maybe a third of it. Pressed Option-1 ....back to a Mill. But there are also times when I can go quite awhile without adding health back at all.
You could always change it to 2 million. 😉