Honestly, I have no idea. But I just spent my lunch hour:
1 - Ghost hunter, instead of going for that one shot attempt, I did it the new way. Worked first try (though for some reason it only kicked in post 9pm in game, not 7pm like I thought it would);
2 - Spectre hunter, after doing the exact same thing as I've been doing for the past two days - joining someone else's world, shooting the windows at the contaminated site the same way, checking to see if the truck lights blinked - all of a sudden it worked.
3 - And yup, I managed to respawn the four hunters in someone else's world (I had previously killed three of them, but one didn't spawn). It was really strange, I took down three, died, and upon respawn, I had to kill all four again. I'm not going to complain though, managed to get it done (along with all hunter masks/ivory keys).
Happy to not have to deal with glitchy spawns anymore, but I hope hunter fights get introduced in more scenarios. They are still pretty frantic and fun (sometimes I drop them so easily and sometimes it's a long, drawn out battle).