I think I found it. Spent quite some time on this so I hope my findings are correct.
Seems like the problematic files are using software parameters. They all have a secondary format.
ie main@L6.1@High
- WMF: Windows Media Foundation
- WMP: Windows Media Player
When disabling WMF for mp4 , the videos (obviously) don't work. In either Movies & TV or WMP.
Turning WMF on, Movies & TV plays the files just fine, HW + CPU
WMP however can barely play the videos. No HW and god knows what's happening to the CPU; It doesn't look pretty.
Likely issue:
Local files SLR player does whatever Movies & TV is doing. 👌
Streaming SLR player falls back to whatever WMP is doing.👎
Lower resolution are probably not an issue and could vary from system to system(?)
Videos without secondary parameters work just fine. ie High@L6
NOTE: Movies & TV doesn't allow streaming from custom URL's. The app has recently had a big update to compete with Netflix and Prime video, maybe prior to this update it was still possible to use custom URL's.
These problem files coincidentally are the same of which WMP doesn't keep an index and doesn't list on DLNA.
Suggestion, Uninstall windows update🤷 or try a fresh windows installation without installing the latest update.