Steps to reproduce:
1. Open attached project "863645.zip"
2. Download video pack (contains 3x1080p and 3x4k videos, link in the edit)
3. Extract the video pack
4. Open "Test" scene
5. Select "Main Camera" in the Hierarchy
6. Enter file URL for each video in the "TestVideos"
7. Between testing Scenario below exit and reenter play mode

Scenario #1:
1. Play two 1080p videos
2. Play two 4k videos
Result: both 4k videos freeze at start and don't play

Scenario #2:
1. Play two 4k videos
Result: both videos freeze on start OR one video starts playing and the other one freezes (happens randomly)

Scenario #3:
1. Play three 4k videos
Result: 0 - 3 videos start playing (completely random outcome) - if the video started playing it will freeze in few seconds

Scenario #4:
1. Play one 1080p video and two 4k videos
Result: 1080p video starts playing, one 4k video won't start playing, one 4k might start playing, but will freeze in a second

Reproduced with 5.6.0b3