If you are using the OnePlus 5T handset then try streaming videos from Amazon Prime, Google Play Movies or Netflix. You will notice that the handset will not be able to stream the videos in HD. Well this issue is not just with the 5T in fact OnePlus 5 users are also facing the same issue.
OnePlus has acknowledged this and are working on to fix the issue. The issue will be fixed via a software update. Many users have already complained on the OnePlus Forum. OnePlus 5T comes with a 6.01 inch screen with FHD+ resolution.
The issue seems to be caused by missing Widevine L1 Certification that is needed to play HD content from DRM services (Digital Rights Management), but that is yet to be verified. Nothing to do with the 18:9 screen aspect as the OnePlus 5 that comes with a FHD screen (16:9 aspect ratio) is also facing the same issue.
If you are using the OnePlus 5 or the OnePlus 5T handset let us know if you can play HD videos from Amazon Prime, Google Play Movies or Netflix. OnePlus 3 / 3T users can also check and let us know.