If I remember correctly, HaT's figures are of a soft plastic, rather than styrene, like the old (and current?) Airfix or Italeri figures from back in the day. Paints adhere with difficulty to that plastic. I ran into that same issue when I was a kid in the 70s and painted Airfix and Atlantic figures. I used the same enamels and acrylics I used on my models-Testor's, Pactra, Model Master. And I noticed that with handling, the paint flaked off very easily.
I know I've seen posts with tips on prepping the surface of this softer plastic, though I don't recall if it was here in the Herd or in another forum. But I know that one old trick was to coat the figures with thinned white glue (eg, Elmer's), aka PVA glue, as a kind of undercoat for the primer. Then apply the primer and finish colors. The white glue provided a little bit of adhesion, that resisted the forces on the figure's surface that lead to flaking.
If I can find the references, I'll share them back here. But I suggest searching on "airfix" and "figures", too. Maybe you'll find similar tips.
Best regards,
Brad