All o2 sensors need to be replaced at the same time. These sensors will wear out and put out data that is slightly off, but still correlate with each other (hence no CEL). Then 1 of the sensors go out completely, you replace it, and it will show how bad the other sensor was.
Kind of like how an engine may have low compression across the board but still run decent because the compression is within an acceptable degree of each other.
You may have 2 worn tires, everything seems fine because they are both even, but once you have a blowout and replace one, then you will notice that it will pull/loose traction/etc on the other side.
You said you tried 2 other new o2 sensors. Replace the other 2 remaining sensors and I bet you it will go back to normal.