Flats inconsistent

I ran 50 flats last night with each filter (LRGB); 5 sec exposures; and all were consistent with the Lum filter; but may were blown out in each RGB series. Does anyone have an explanation for this?

I ended up deleting the overexposed ones before processing in Nebulosity.

Camera: ZWO ASI1600MM cooled; L (1x1); RGB (2x2); High Dynamic setting (Gain 0) and 5 sec exposures.

Thanks!