I was having this same issue with my Samsung HMD Odyssey and was able to come up with a fix.
I created a bug report (with fix linked) here: Windows Mixed Reality device screen distortion (SteamVR devices unusable besides ) - XR Development - Epic Developer Community Forums
Please upvote it to get this fixed ASAP!
Link to my pull request to fix the distortion issue in 4.18 here: https://github.com/EpicGames/UnrealEngine/pull/4354
If you want to build the engine from source, this will fix the issue and get it working before Epic Games fixes it.