Transport vulnerable to ground units sometimes, is it a bug?

T2 transport sort of landed when it came to rescue an ACU
I expected it to be vulnerable to my ACU. I turned off auto-overcharge, waited for the transport to stop, and then I manually overcharged the transport to death
Is that a bug? An exploit? Or is it supposed to be that way? When are transports supposed to be vulnerable to anti-ground weapons?

It is supposed to be this way. The simulation is just a physics engine. t1 Artillery can hit t1 interceptors too if you're unlucky. Time your OC right and the transport will be shattered - as intended.

A work of art is never finished, merely abandoned

OC of transport is something I learned of a few days ago as well. Have yet to use it in anger.