Do Plaguelords have infinite cards in their deck?

I played the new adventures first chapter on heroic and managed to build a kinda consistent Milldeck I made it to the boss with.

But trying to mill him it happened on 2 occasions, that almost before I got him running out of cards, he just got 20 more cards added to his deck. I had not played any card adding cards to his deck, nor had his heropower or any permanent effect that was mentioned anywhere left a note on WHY he just got his deck stocked up. That's really frustrating, and so I want to ask here:

Why did the Plaguelord of "Tombs of terror"'s first chapter get aproximately 20 cards added to his deck multiple times, and what are these cards?

Solution 1:

Each Plague Lord has three phases, one for every 100 health. When the Plague Lord passes 200 and 100 health, it gains a new Hero Power, a new deck, and draws a new hand. In a way, each one is three boss fights grafted together.

The good news is that, because of their persistent health mechanic, the next time you encounter that Plague Lord they'll already be in their second (or third) phase, meaning they will gain fewer cards over the course of the encounter.