I think freeze actually went over this in one of his videos.
The attacks that JJ throws are considered zone attacks and I think they lack a parry multiplier on stamina. This keeps one heavy parry causing OOS.
Hito heavy or charged heavy follows this rule as well. Not because of zone property but it keeps one parry from putting them out of stamina.
I think the fact that the JJ's heavies cost 60 stamina each is punishing enough lol. 2 heavies and you have to go into Sifu's Poise. Beside, I think Stamina regeneration is his whole thing anyways so maybe it makes sense that he doesn't lose any when being parried. His character description says "Stamina Regenerator".