Yes, your Bias Frames have a higher mean and median ADU value than your Master Dark Frame. That alone indicates something was amiss during the capture of your calibration frames. You probably should check the gain and offset settings for everything in your capture program. I suspect something was set up differently for the capture of Bias Frames, Dark Frames, Flat Frames, and / or Light Frames. A difference in gain or offset are the most common difference. A significant temperature difference between the frames can also result in mismatched calibration frames.
Such a difference in gain, offset, or temperature can cause loss of background levels when you attempt to use frames of mixed capture parameters. It can severely clip the background levels during calibration and cause both a darkening of the overall image as well as the "needle" histogram you are seeing.
To measure the mean or median ADU value of an image, I use the Statistics Process tool in PixInsight. I do not know which image processing programs you have available to you and use. Often, the capture program can give you a readout of the mean and median of an image. I use Sequence Generator Pro for capture and it also shows all the statistics needed to sanity check an image. Look around in the tools you use to see which ones report on the statistics for an image.
It is always a good idea to sanity check images using statistics. Your Bias Frames should have the lowest ADU mean / median followed by the Flat-Darks, Darks, Lights (target image), and finally Flats. If your "average frames" do not show increasing ADU values in that order then you know something is off in the capture of the data and calibration problems will likely result.
PS: Regarding capture of Bias Frames, many capture programs will translate between what exposure time you enter and the capabilities of the camera. For Bias Frames, I always enter 0 for exposure time. Sequence Generator Pro / the camera device driver then uses the shortest exposure possible for the camera. Maxim DLE, and Nebulosity used to work the same when I used them.
Edited by jdupton, 18 July 2019 - 08:39 AM.