Are your DO meter readings accurate? The DO meters I used in laboratories had a built in stirrer because they did not work properly unless the water was circulating. I'm not sure what type of DO meter I was using or whether chemiluminescent DO meters need moving water to work properly.
The DO meter is in the glass carboy as it is being filled by pump, so it is always in well circulated wort. This is a very high quality Hach LDO. Even if it was off a point or two, doesn't change the trend of sudden oxygen desaturation, of a high gravity wort, in very short period of time. I will run this experiment over the next several brew days to confirm this finding.
My current message to myself on this - I should pitch yeast immediately upon completion of wort aeration/oxygenation. My previous method was to fill the carboy, stop and do a yeast count, calculate my pitch volume, then pitch the yeast. This all the while the wort was desaturating....