Looks like this issue is either specific to SDL/OpenTK or Hammerwatch then. I've searched around, but I can't find much information on issues like this where the drivers weren't the problem. It could be something in the code HW uses to create/manage the graphics context, but I have no real way of accurately debugging that without the actual source (well, it'd be difficult.)
Hoping a dev can chime in here and maybe do some tests or provide a different game binary with more debug output enabled.