At the end of The Circuit Void Flood mission, I closed all the ruptures and the objective changed to "Defeat All Thrax Attackers". It appears to have spawned the said Thrax below the map (there is an enemy indicator on minimap with arrow pointing down) which prevented further progression.
Not sure what other information is helpful, but this is played solo and does not relate to host migration-type issues.
Edit: Screenshot https://imgur.com/gallery/ihc4ar1