Monsters have 100% Block from Expedition is uncapped and blocking 100% damage.

You are incorrect. Monsters correctly have maximum chance to block attack damage, with a base value of 75. This does apply. With the total mods shown in your screenshot, the monsters will have a final value of 85% chance to block attack damage, because the +100% is limited by the maximum, which has a +10 bonus.

Checking this has led me to an actual bug though - monsters currently don't set a value for maximum spell block, which means they have 0, instead of the correct value of 75. This doesn't mean their block is uncapped, it means the cap is zero - with the total mods in your screenshot, the monsters will only have a total of 10% chance to block spell damage. I will get this fixed.

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Ranzear wrote:

Any comment on monsters taking zero damage when they have such high block though? That was supposed to be changed back in Legion.

No, specific legion monsters with innate high block were given a glancing-blows-like modifier where those monsters didn't block all damage. That was not a systematic change and has never affected all monsters.