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What do you have the MacBook Pro plugged into? Are you using the Apple AV Adapter with HDMI? If so, I had the EXACT same issues with my LG 29" UltraWide display and said adapter. Ultimately I couldn't find a fix for it.
 
What do you have the MacBook Pro plugged into? Are you using the Apple AV Adapter with HDMI? If so, I had the EXACT same issues with my LG 29" UltraWide display and said adapter. Ultimately I couldn't find a fix for it.

At the time, it wasn't plugged into anything.
 
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