Do you have the same issue if you follow the OP’s steps to reproduce it?
• Yes. Exactly the same.
Provide extra information to complete the original description of the issue:
• Can be reproduced also with weather that changes the outside light. With more clouds and rain its getting darker while daytime and the display of the GNS530 and 430 starts dimming. I also use a DC3 retrofit. A Beechcraft M18 is still missing in my list (i purchased it in MSFS2020) So i can not tell if the issue exist with the Twinbeech too.
It seems that this issue is connected to how much light is outside and that the cause (night/ clouds/weather) do not to matter.
If relevant, provide additional screenshots/video:
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