Had 2 x 4* from Invasions too. Tiger I and Stug.

Would be surprised if 3* guaranteed and 4* possible was a fixed feature.
Under normal circumstances would be reward spam compared to the other sources for 3*/4* in the game.

But who knows, can of course be that normal does not apply. Could think of a few reasons why it might not (PoC numbers suggesting 20% player drop in a month being one).