Syndicate: Cannot trigger Intervention after fully clearing the map

This just happened (and how you can reproduce it):
- The map had Syndicate.
- I activated the Delirium mirror and went to clear the map.
- Encountered Transportation (could be Fortification just as well) nearby, defeated it, but didn't interrogate the Syndicate members, as I was in a hurry to clear the map in the fog.
- Finished clearing the map, 0 monsters remaining.
- Went back and interrogated the Syndicate members.
- Did the Research encounter.
- Can't trigger Intervention, there are 0 monsters left.

Workaround: brought The Writhing Jar, spawned and killed the worms, Intervention finally triggered. But I'd rather see Intervention trigger in some way that didn't require such exotic solutions.