Why Garmin Research&Development not fix this bug!!! Why Why Why ??
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NekoRunner Have you read the last post on this thread from over a year ago from jvoclv? It is not a bug. It was a result of user applied settings.
It is a bug actually.
I have the same problem (running activity defined with both auto climb and it's color inverting on) and the display is changing to black randomly during activity when running on flat surface, no ascent, no descent whatsoever (and none recorded by Garmin either).
In which case the usual question "Have you reported to Garmin?"
The post I was referring to implied that it was a user setting that resolved the OPs post. You've now provided more specific detail that might indeed be suggestive of a bug. Hence the suggestion that you discuss with Garmin.
I've noticed the same issue today on my 6X after I ran by a nearby Strava segment. The Strava segment notification came up several times (I didn't go that way), then I noticed that the data screen had changed. Also in the process the background color had changed to black and after some time back to white. It changed to black once again at the end of activity.