I had this same issue for a long time--videos play fine in edit mode, and only play over HDMI/extended if the presentation is duplicated, not using Presenter View.
Most sites suggest disabling slide show and hardware graphics acceleration under advanced, but in my case, with an RTX 2070, playback of simple PowerPoint animations is horrendous--although videos do play for me when doing this.
I solved this--and I honestly think I'm the first to do so--by doing the following:
- Windows 10 x64 connected to projector via HDMI
- Right click on desktop
- Display settings
- All the way at the bottom, click Graphic Settings
- Choose Classic App
- Browse for PowerPoint at C:\Program Files\Microsoft Office\root\Office16\POWERPNT.EXE (or wherever your version is located)
- Click Options
- Change to High Performance
- Restart PowerPoint and try your video in show mode again