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Really all the black point is indicating there is that there is some sort of stretch going on, so you may not be able to tell by looking at the histogram if the levels are good for a flat.
To compare apples and apples, this is the histogram of a flat I shot with my ASI2600MC, two histograms here both presented with a log view rather than linear so they present the same way yours would. One is with no stretch applied and you can see it looks good, all data is well away from both sides of the histogram, and the second is the same flat frame with an auto stretch applied. If that was unstretched it would indicate a bad flat as you would say that the red channel is clipped, but it is really just that the black point has been moved up in the auto stretch.
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