Object placed above MRUK furniture "jumps" up/down when pushing right thumbstick

Context
Unity + Meta XR Building Blocks. I'm building an AR app (Passthrough + MR Utility Kit). I show a world-space UI dialog above an MRUK table (Canvas in world space, RectTransform placed at the table center + slight lift to sit on the surface -> all via script).

Symptom
Whenever I push the right controller thumbstick downward, the dialog appears to "jump" ~0.5 m up, and pushing again makes it jump back down. This happened both on the device and in the Simulator.

What it actually is
It's not the dialog moving. Logging showed Camera.main.transform.position.y toggling between two values (1.047 <-> 1.547), while the dialog's world Y stayed constant.