No, I wouldn't say that is, "Normal". DO levels will drop by about 10% every three feet due to hydrostatic pressure, but your drop is way more, not to mention your pH.
From what I know about your BOW, I would guess your contrasting pH and DO levels are the result of muddy water. All your photosynthesis is happening within the first foot of water, since there is no sunlight past that point. The plants are using the carbon dioxide, which acts like carbonic acid in water. CO2 removal reduces the acidity of the water so pH increases. Decomposition at the bottom of your water has the opposite effect and is decreasing pH and lowering DO.
I would "think" more aeration but I would like to see more tests prior to suggesting anything.
This still seems accurate to what is going on in your pond. Not sure why you returned the mud bugs? They are called mud bugs for a reason.
I agree your bloom is now generating oxygen at the upper parts of the water column during the day. Also, I think your pond is cycling efficiently at the upper part of the water column. The pond isn't infertile, it is just not overly fertile. Your bottom may have become somewhat anaerobic? If so, I would expect to see similar results for quite some time.
Do you know how much muck you have on the bottom of your pond?