Not finding DRK squishy at all.

[BadRNG]

No you're wrong, I tested it and there's no difference between tanking ability before and after lv50, there's no "special new way of calculating dmg" in HW.

Lv 50 PLD and lv 51 PLD take the exact same amount of dmg if they 're sharing same DEFENSE attribute.

Did you consider that the 50-51 jump might not be as large? Was the PLD 50 and DRK 51 in your tests to match?

I do believe it's stat weights causing this(it's been widely reported as happening to old jobs too), but regardless of whether you believe that, the only sane thing to do is to test this on two characters of same stats AND level, ideally 60. There would be no room for debate there. That's likely how SE would test it as well.