Screen dims when VLC loads file and second monitor is attached

Hello,

recently a user reported an issue with my app, which uses VLCKit to play IP cameras video streams: he's using an iMac 5k running macOS 10.14.6 with a second monitor attached and, when he launches my app, the screen brightness automatically decreases a little.

Googling for possible causes, I've seen that this issue has already been reported in the VideoLAN forums with no mentions of a solution yet.

I did ask my user to test VLC.app and indeed he confirmed the dimming also happen with VLC Media Player.

Both screen dim brightness, if this helps troubleshooting.

Any suggestion for a possible cause and fix?

Thank you!

Cesare