Just tried them here on An 20.0.1 and see no obvious problem. Hints work as expected.
Make sure you have 'snap to objects' ON when you try to attach a Shape Hint.
If they still refuse to work, try a different file, different version of Animate and so on until you isolate the cause.
I have noticed many issues regarding shape tweens even with the added benefit of using shape hints. It's a voodoo science for sure and even with the simplest of shapes, shape tweening with using hints most often fails for me. It's never been 100% reliable and with the latest release I do think it might even be worse. I just tried to shape tween a square between 2 keyframes using shape hints on all 4 corners and it produced an animation that was very unexpected.
I'm also trying to animate a character that uses Layer Parenting and strokes. One stroke per layer (2 vector points) and often the stroke flips upside down even when only shape tweening one point across 2 keyframes and when I try to apply shape hints to the stroke, it never snaps no matter what.
If you can provide a screenshot maybe we can suggest a specific work-around based on what you are trying to do?
thanks, maybe it's only a "snapping" problem