Test stage 5-1-6, blow job ticket. It seems to me that Nonna restores health to infinity. I can't provide a screenshot yet.

I completed 5-1, so I can't recreate.  I did fight the zone cleared 5-1 Nonna and didn't see a problem, but that doesn't prove anything for your case.

I'd file a bug report once you can get a screen shot.

I'm sorry, but I forgot to make a screen.
I really have already passed this stage. Nevertheless, this problem is still relevant. Because the levels are too hard!