Having heard a few times that there's no DEF down cap in Nutaku version yet. But AFAIK, the DEF down cap is always there in DMM version, it's only the ATK down cap that came much later.
So I run this little test, for this matter is important when using type C debuff.

1st run: 65% DEF down from Amon, D'art and Cthulhu.
Test of the DEF Down cap-1.jpeg

2nd run: 50% DEF down from Ambush, Mordred and Cthulhu.
Test of the DEF Down cap-2.jpeg

Clearly, the DEF down is capped at 50%.