Abyss Scarab of Edifice occasionally does not spawn Stygian Spire in Toxic Sewer map

Have run many dozens of T16 Toxic Sewer maps with the following 5 scarabs:

2x Abyss Scarab
2x Abyss Scarab of Multitudes
1x Abyss Scarab of Edifice

Abyss Scarab of Edifice states:
Abysses in Area that do not lead to an Abyssal Depths lead to a Stygian Spire. Abysses in Area spawn Hoards as they travel.

Other possibly relevant information: I also use Ambush map mod and I use the Searing Exarch witness.

Current strategy is to pop an Abyss and ignore Hoard piles until the Abyss is completed, backtracking quickly to click the piles. That way it lets me focus on killing all Abyss monsters that pop.

I've had maybe 10 maps so far where some Abysses spawn and travel a couple of times before eventually ending up in its final spot where it spawns monsters, you kill them, and it just abruptly closes leaving a Hoard pile. No Abyssal depth and no Stygian Spire. Abyssal depths are uncommon, but if they do not appear, a Stygian Spire should have appeared instead, but they do not.

I thought I was maybe missing too many monsters at prior spawn points so I started to full clear the Toxic Sewer map before starting any Abysses to prioritize them. Even when I full clear the Abyss, it occasionally does not force a Stygian Spire, leaving only the Hoard pile at the very last spawn. I also backtrack as I run back to click all the Hoard piles and I do not run into any stragglers at all as they have all been killed before.

I have noticed that many of the Abysses tend to path in similar directions and often end up having their final spawn points in the same room. Given the layout of Toxic Sewer, this is pretty common. Don't know if this has an effect on this issue of Stygian Spire however.

Edit: I was told that occasionally the very bottom slot after you unlock all 6 slots in the map device may be bugged. That is where I place my Abyss Scarab of Edifice. Not sure if this is relevant. https://www.youtube.com/watch?v=Ct4_REJDDgo