This bug is a doozy so be ready for a bit of a read: Felix's Q seems to have trouble hitting people at different elevations than himself.
I did a lot of testing with Celeste R, an ability which shares a lot of similarities with Felix's Q when it comes to interactions: Both are large AoE areas around the player, and are affected by line-of-sight. In the Abyss, Celeste R will largely hit targets no matter their elevation below the ground plane, whether she's lower than the enemy or vice versa. Felix Q will not hit targets if their elevation is drastically different, and seems to completely ignore abyss balloons.
In the attached video to this message, you can see Celeste R hitting the ground-level enemy while Celeste was mantling during R. Felix's Q does not hit these same targets, even with much more generous elevation differences (I had to move slightly to the side while testing Felix because Felix L SHIFT does hit the dummy while mantling so it caused trouble in the recording.. L SHIFT hits but Q doesn't?)