Got Herald but no Diablo in TZ CS

Diablo did not spawn as expected in terrorized

Chaos Sanctuary tonight.

I've double- and triple checked that all seals are open, and there is nothing left of the seal boss packs as far as I can see. So I'm trying to figure out what's going on.

My procedure: I went counter-clockwise through the seals, however, before spawning

Infector of Souls' pack a

Herald spawned, who was itself a Venom lord (with pack). After killing them I spawned Infector's pack, killed them, then went on with

Lord De Seis and finally

Grand Vizier of Chaos. And opened all seals along the way of course.

Then nothing more, no "Not even Death..." message, no Diablo.

Ideas about what could've happened:

1. I'm not entirely positive I killed every seal boss, I only cleared all visible monsters in the packs - I'm running a lvl 90 FoHdin with lvl 25
   Conviction so packs disappear pretty quickly (P1 online).
2. Could it be related to the
   Herald, maybe it somehow replaced
   Infector of Souls so that he did not spawn with his pack? Seems unlikely, but still an hypothesis.
3. Something else with the
   Herald mechanics messing things up?
4. Maybe Grand Vizier flew out and hid above the lava, somewhere out of reach? I did walk the Edge of his entire wing of CS, hurling holy
   Bolts out in the Void, without results.

When searching around I found a thread about Dclone sometimes preventing Diablo to spawn, but that seems still different.

Has this happened to anyone else?