Chrome Version: 58.0.3029.21
OS: Pixel (likely all devices)
VRCore: 1.3.149854881

What steps will reproduce the problem?
(1) go to https://webvr.info/samples/03-vr-presentation.html 
(2) rotate phone so content is in portrait mode. (phone is held in a vertical position)
(3) turn phone and hit reset pose.

What is the expected result? content is always centered when hitting reset pose. 

What happens instead? At some point if repeating step 3, the content is centered behind the viewer. Does not happen every time, but is easily reproducible.

Note that this does not occur when the content is in landscape view mode. (Phone is held horizontally.)