That may be true, but when looking at other teams this didn't always seem to make sense. In this example, Mich State has said A+ before he even got to the final four. Or if you look at UC:
https://www.whatifsports.com/hd/TeamProfile/History.aspx?tid=4264
They have been at A prestige for several seasons now, including after a final 4 --- yet they will be A+ after a back to back second round exits this year? That seems strange.
That said, you are probably right, as I realize prestige looks at more than just the previous year - so maybe Mich St. has such a strong baseline prestige that just getting into the NT has vaulted him to A+ again since it looks at several past seasons results.
I am probably just a little salty too, since that means I am stuck at A- again even after an Elite 8 appearance.