Abyss is sometimes uncompleteable

Title. I'm talking fissures in maps, which don't complete towards the points where monsters will erupt from.

Second time it happened. 0 monsters left in map.

Bug Report ID: #4105639605

First time was on Riverhold, I think on a dam or something, I believe something about the narrowness of the map prevented mobs spawning or similar. I don't have the report ID for that.

This time Abyss spawned on top of an Expedition, where I think placements of markers/remnants/chests blocked it.

Happened again.

Bug Report ID: #875627689

This time, 7 monsters remaining, but they are nowhere near the abyss.

Happened again.

Bug Report ID: #912117235

Happened again.

Bug Report ID: #3650016493

I believe this time, it's clearly because a Ritual altar was placed on top of an abyssal... "hole".