Load several flat mono videos (3-5 will do) to headset aspect ratio doesn't matter but 16:9 more obvious.
Open video in DeoVR - in playback settings transform set screen size to 100, curve to 0, height to +14 and set as default. play next video advancing from controller or from playback control panel, not from filelist (important) applying default transform, repeat for all videos.
Advance to a video where custom default transforms were applied - aspect ratio will be squeezed to something like 3:4
Notes: further investigation
It seems when switching videos (or images) in folders (previous/next) via controller or playback controller panel the video always plays back with the incorrect "squeezed" aspect ratio. This can be corrected by apply the transform defaults or clicking twice on the flat or mono buttons on the control panel.
Opened videos/pictures directly from the filelist usually display with the correct aspect ratio.