Android 6 is quite old and likely is the cause of the problem. The only workaround will be to go to UI Settings > Video Player and change the H.264 player to JavaScript. It is less efficient which sucks for a mobile device, but inefficient is better than broken.
Alternatively perhaps you could go to the Digital Zoom section of UI Settings and turn on browser native zoom, that way a totally different zooming method is used when you pinch and it should not be affected by the bug.
Alternatively perhaps you could go to the Digital Zoom section of UI Settings and turn on browser native zoom, that way a totally different zooming method is used when you pinch and it should not be affected by the bug.