The T-6 II uses a on-board oxygen generation system, or OBOGS that has had problems for a number of years. The AIr Force in 2018 undertook a "total rework of the OBOGS with a revised software package, and changes in maintenance routines, could take up to four years to fully implement – and there still might be more work to do afterward."
That quote is from the below article:
Air Force Says It Knows Why T-6 Trainers Are Choking Pilots, But It'll Take Years To Fix
Also from the article
“So far, technical efforts to date and analysis of data collected have determined that pilots have been exposed to significantly changing levels of oxygen concentration,” U.S. Air Force Lieutenant General Steve Kwast, head of AETC, said in a statement on Sept. 13, 2018. “The varying levels of oxygen concentration, even though in excess of what the body typically needs, has caused physiological stress that most pilots on most days actually adapt to without noticing.”
If they were down low it wouldn't be hypoxia per se, but it could be that they were getting "bad air" from the OBOGS that caused them to be impaired.
Impairment of the crew would explain no Mayday or emergency call on guard or Center (queue the indignant proper radio phraseology crowd argument here) and it would also explain the lack of ejection from such a high perch. I have no idea the Navy position on the Texan OBOGS but would bet they followed Beech's lead from the USAF fixes.