Unique Boss Accompanied by a Harbinger sextant mod not working on Grave Trough map

Bug #: 3,984,243,018

Sextant Modifier:

Unique Boss Is Accompanied By A Mysterious Harbinger
Unique Boss Drops Additional Currency Shards
Harbingers Drop Additional Currency Shards

The first few uses of this sextant modifier worked in other maps but when I ran Grave Trough the harbinger didn't spawn. However the Unique Boss did drop currency shards. The only other map mod that might have had an effect was a "Area Contains Two Unique Bosses" mod.

I have a screenshot but it was taken after the boss kill and only shows the map mods and loot so I don't think it will be helpful.