The A-10 flys vertically upwards after firing.

I saw this happen and am not sure if this is a bug or what they actually do IRL.
It looked pretty odd.

I noticed this too. Pretty sure at some point it would stall but who knows.

It's how the A10 operates. It starts around 25,000 feet, then aims downward almost vertically to target, fires a burst, and then pulls up again. It's made to mitigate exposure to ground fire, and minimize loss of ground speed when firing (the gun will actually slow the aircraft significantly in level flight).