If I look down at an angle, such as mining a block in the ground, I see my body, but as soon as I am mining a block straight ahead, the model is hidden. This angle seems to be exactly 30 degress:
29.9 degrees:

30 degrees:

forge 1.17.1

Seems like the dynamic mode applies even without vanilla hands turned on, that's a bug. Just turn off Dynamic Mode. Also, the model doesn't disappear, it just moves the arm down out of view(and then hides it at that angle), so usually the vanilla hands can come up.

i got this issue the same