As critical as I am about the vaccines, the JPs probably all took them too and aren't being plagued with these same health issues for the most part. The vaccine could be messing up their immune systems yes, but a shitty diet and lifestyle is going to exasperate any problems they have.
I think it's likely their lifestyles being absolute dogshit combined with the amount of exercise they do for idol shit. Western junk food is hilariously fucked and wouldn't be surprised that the Japanese junk food is somehow more healthy and isn't completely annihilating the JP talent that are slobs. However on the topic of the vaccine, it is important to note that Japan did reject 1.63 million doses of the Moderna shots due to contamination after they found 39 vials at eight vaccinations sites were contaminated. This would lead credence to the rrat that it's only a select amount of the vaccine is a poisoning and would explain why not everyone is dead despite them all taking the cloutshot. Additionally Japan rejecting the Moderna shots would explain why JP talents are largely unaffected. Again, it's likely just lifestyle issues but it's fun to rrat every once and a while. Please don't shoot me, Jannies.