Trial of the Sekhemas - Hourglass Trial - Rare monster fail to spawn

I've encountered this a couple of times now.

Level 92 Lich with 6x Storm Mages, 1x Terracotta Warrior Spectre, 1x Skeletal Cleric, 2x Skeletal Warrior. Sceptre and Buckler on main weapon, Chiming Staff on offhand, using different passives for different weapon sets.

1. Pull the hourglass to begin the trial.
2. Immediately cast Sigil of Power, followed by Conductivity supported by Cursed Ground (switches to weapon set 2, despawning all minions)
3. Cast Parry to switch to weapon set 2 (summons all minions)
4. Cast Pain Offering followed by Unearth

Outcome: even after searching the entire area, no rare monsters found. Normal monsters spawn but slowly, and eventually the timer runs out.

I have noticed that sometimes rare monsters *appear to* stop spawning after the first 1-2, but this might just be because I couldn't find them behind the adds and particle effects going off.