Nameless seer do not spawn

Bug number - 1795335141
Map - Dunes
Realm - London
Time - right now

I should get Namless Seer on the map after killing last pack of mobs, as on the screenshot. There are 0 monsters on the entire map, i've run all around 4 times and he didn't spawn at all.

Additional info - during clearing, just after killing the boss (+/- 50% of mobs were killed on the map) i've left the area for trade, and came back. Maybe this break the spawning process?

Or maybe it's related to "Back to Basic" keystone? But this would be strange, as coffins and allfalmes are working....

Screenshot:
https://imgur.com/74GFt4I

Just did T16 Dunes and I got him as Devoted mod. It spawned for me. I didn't leave the map to trade.

P.S - I don't have Back to basics activated. That could affect it.