Ever since I started playing Minecraft VR on my WMR headset (Odyssey Plus) a month ago, I've had this issue related to my hotbar:

Sometimes, when I press the right trigger button to use the selected item (either for mining or hitting a mob with my sword), the selection switches spontaneously to another item of the hotbar. There's no specific pattern or condition for this bug; it happens randomly and quite often, sometimes when mining, when fighting mobs, or even when hitting the air. The only workarounds I've found are to keep hitting the air until it stops, or to reorder the items in my hotbar.

It used to happen in 1.16.210 version and it still happens in 1.16.220.