Looks like Samsung got their specs wrong as well as shipped with the wrong wall charger for the Tab A8. The AFC in my package is a 9V 1.67A brick, which is 15W, and it cannot power the device. Can't even power it from 0 to power-on. So I got in touch with the tech dept and they cited the same figures: AFC 5v+9v is the correct charger. So I figured I had a bad brick and bought another. Nope, same result.
Tab A8 requires the 25W 9v Fast Charger, same as the S20 S21 and S22. It has sufficient amperage to provide the 9V needed.
For reference, Samsung's Fast Charger is also adaptive, and is rated:
25W / 2.77A = 9V
15W / 3.0A = 5V