No video, just black screen, when connecting external display.

When viewing video from iview when an external display is connected to my MacBook Air, it only plays the audio, the video is black. (see attached screenshot).

The video is black on both the external and internal displays. If I unplug the external display's HDMI table, then I can view the video on the laptop's display.

There are no such issues with Firefox or Chrome -- it all works correctly with these two browsers.

Is this a Safari or macOS bug?

[Edited by Moderator]