It seems like greased line is not rendering correctly in webxr when multiview is enabled. You end up with one eye rendering correctly while the other has the mesh moving around the view with you. This is also the case for some other shader materials I’ve looked at.
It could be related to this issue where there is just a typo in the shader code?
cc El Maestro: @roland
Master, give me a bit of time! I’m in the middle of juggling three things at once, this one Babylon.js meets Google Maps and Mapbox!
and a dynamic Navigation debug extension for the new Inspector, plus my daily work… 
The fix looks pretty straightforward. Thanks, @dsaffo, for finding the topic in the Bugs section, that really helps! I just need to wrap up at least one of the things I’m currently working on first…