Looks like my question did not make it to the official FAQ.

Sitll, I wonder what other people think?

Undying question:

If I have a group limit of 0 Minion Ogre and 1 Master Ogre, which have the undying ability.

When it dies, I replace it with a Minion ogre ignoring group limits.

Can I respawn a Master ogre on the next re-inforcement if the Minion Ogre is not dead?

And then, if the Master dies again, it mean I can replace it with another Minion over and over, creating an infinite loop of minion spawning…

Bagdad said:

I can see how it might be taken that way, but I would say no.

I would say the Master cannot be reinforced until its replacement is dead. I presume from its name that Undying is not supposed to represent it becoming a different monster, but rather that it's hard to put down. That suggests to me that a minion placed to replace an Undying master is still considered that master for purposes of reinforcement.