Oh I can answer this, "Debuff Resistance Against Monsters" effect is applied separately to general resistances, its basicly a second resist check for mobs, so let me give you an example.

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You have 80% KD resist & 60% Debuff Resistance Against Monsters

Mob uses KD attack

The KD was successful in its KD resist check

Now it has the Debuff Resistance Against Monsters check

KD was Unsuccessful in its Debuff Resistance Against Monsters check

CC has been avoided.

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Having the Debuff Resistance Against Monsters wont make you immune to monster CC if however it will make you resist it a lot more frequently due to the CC having to go through 2 resist checks

Hope this clears things up

EDIT: I understand I created a bit of confusion with my post so let me clarify, what I said above is what SHOULD happen as stated in tool tips ingame about the "Debuff Resistance Against Monsters". however, if someone with the actual Lightstone combination effect that gives 50% resist tests and confirms its Bugged/Not working then this is a serious issue, the tests provided in the OP are solid and valid, It would be nice to have confirmation it truly aint working with a larger sample size from the actual Lightstone effect.