Damage display bug. Someone captured this on video for me, so I'm borrowing their vid (with permission) to show you the bug.
@~1:56 Anon here takes a shot into the side of an enemy KV-4. At the time he fires the round, he's zoomed. Before the round lands, he zooms out. The audio says "Penetration" and you can see the HP bar has reduced in size, so the hit was counted correctly by the server. However because he zooms out before the round lands, you do NOT see the damage roll indication above the enemy tank.
I'm not sure if this is new to 2.5 or old. FWIW, it happens to me in all manner of tank types/classes/maps. The key piece is managing to zoom out after firing the round but before the round lands on the tank.