When moving the camera in VR, BillboardGuis can appear to ‘jiggle’ around their position, as if they were being snapped to pixels. You might have to look at the above clip closely to see it - it was incredibly hard to see on a flat display, but in the headset it is incredibly noticeable due to the way human vision stabilises things.
This can be reproduced with any billboard UI - the contrived setup shown above is only so that I could stabilise the video with editing software. I recommend you use a VR headset to repro this.
Expected behavior
I expect the BillboardGui to remain perfectly still and locked to its position in 3D space.