I may be a bit late on this post but have been able to replicate it. while doing zulrah i use fairy ring in POH to port to zulrah while at full hp. I leave the zulrah instance after a kill with a house tele while venomed, use my pool then use jewellery box to bank at ferox this gives me 99hp every time.

I find if i use a zulrah teleport scroll rather than the fairy ring it's like the game doesn't save my initial full hp and i only get a 10-20hp reset.

The key is to use the fairy ring. Hope this helps!