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Glen. If you are applying a bias frame to your darks and/or lights, you have most likely truncated your data. Your experience and image tends to indicate as much.
Basically, I now avoid 16bit processing methodologies with DSLR data - it's not linear.
Suggest you try the following;
1. Stack your darks as is, with no pixel rejection
2. If you had flats for this image take bias frames and stack same as darks.
3. Again, if you had flats - calibrate with master bias
4. Calibrate your light subs with your master dark and master flat (if you had one) only - the bias is in the dark.
I have found processing in this manner consistently reliable, maximizing the available data. It manages the non-linearity caused by in-camera processing of RAW data, which is unavoidable.
Good flats make a difference. If you get around to taking flats, and you really should, pixel rejection is necessary as a rule. The typical method of combination is average/multiplicative using a Sigma pixel rejection. PixInsight terminology, but you get the drift.
A starting place... you may be able to improve on the process.
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