Found them from stations in HIP 20492 (also present in HIP 20850, more on that in a sec.)
Issuing RX Reactivation Missions for ground sites in HIP 21261 (Thargoid Controlled system, ground sites had Revenants.)
What may be more useful to you (especially since the Thargsday ticket is coming up,) is how I found them.
Used the Thargoid War information pane on the left hand side of the starmap.
Then went to each of the currently listed 'top 5' Alert systems.
Found AX Reactivation missions available in the 3rd system on the list.
They are listed on the Operations, not the Thargoid, area.
If they have you go to a system that is in Thargoid Recovery status (as opposed to Thargoid Invasion/Control/Alert), there will not be any Revenants at the ground site.
As I was upgrading some ground gear at the time, I also noticed they were being offered in the nearby system of HIP 20850 which is NOT in any Thargoid war status, and all the missions being generated were going to the same system (HIP 21261.)
I think the way these missions are generated is that specific Thargoid war status systems (and definitely not all of them) are being flagged for ground recovery efforts.
Then, human controlled systems within a certain radius of them are allowed to generate these missions if they are in Thargoid Alert or non-Thargoid status.
What's weird is that I haven't found any of these missions in systems in Thargoid Recovery status (which is where they previously generated before Revenants were added.)