Azmerian Wisp not spawning (but Delwyn does)

I had this happen about 15-20 times now.
Delwyn spawns, the hunt music starts playing, but there is no wisp.

I have the overlay map on 100% of the time so I'm positive that there is NO wisp spawning.

Basically, I can't interact with the mechanic.

When this happens, are there any rares left on your map? I suspect it's because there's no rares for it to possess that it does not spawn.

I wrote a bug about that here: https://www.pathofexile.com/forum/view-thread/3757257

Yes, there were rares left. I killed them after I encountered Delwyn,numerous times.
So I think maybe even your issue may not be related to only no rare mobs being left but also to the whole mechanic being bugged beyond belief.