I cast it on 3 beetles and ogre on Lion's Way. Placed it so that all of them were inside the circle but not targeted to any creature. The ground was uneven. I think the ogre stood on higher ground and the beetles were half way above the placed circle (so their feet and lower torso was below).
The ogre showed saves but the beetles seemed unaffected or at least didn't show saves. The ogre died, the beetles I had to beat up.
Beetles have died from phantasmal killer before so they shouldn't be immune. It isn't the first time something similar has happened with this spell but since earlier the saves weren't showing so it was impossible to say if they saved or not. Sometimes things died and sometimes not.
This time I was spesifically testing the spell so could pay more attention to it and have less fear of dying in a heat of a battle.
I've ran into the same with dismissal and banishment on an occasion. That nothing happens. Perhaps due to the placement of the spell and not being on flat surface but I've shrugged it off as potential hiccup/lag/something.
I did a few more test casting with a group of goblins. Tried it 3 times.
1. Somewhat flat surface, saves showed for all in the circle.
2. Side on a hillside, all goblins in the circle showed saves.
3. Again on a hillside but the center of the spell placed higher than where the goblins were. 1 goblin rolled saves, 1 goblin ignored.
A little random. Would need a few more tests.
4. Same goblins. Tried to place the center of the spells above them. All rolled saves. 2 potentially ignored but I think they were outside the circle. That is if the AoE is the exact circle.
5. Did again with just 1 goblin. Placed it considerably higher. Save was rolled.
6. 3 goblins. Even ground. 1 rolled saves. 2 didn't. I hit each once but it showed no damage on the logs and they just died.
7. 2 kobolds. A bit uneven ground. Kobolds were on different levels horizontally. No rolls. Both just died.