Hi Bob,
It seems that somewhere in the Factorio code, someone did something to the projectile/predict movement code. This results in completely wrong directions of the plasma balls - like the opposite direction the enemy's are - as soon as your tank is moving.
Sometimes it is OK (most likely when enemies are in the north and you driving south), but most of the times it shoots in more or less random directions. If you stop the vehicle, it seems to shoot in the correct direction.
Beam weapons and stationary turrets work as expected.

I have the same issue. The Vehicle Plasma canon detects a biter approaching, but then fires 180 degrees away from them. Occasionally it shoots in the right direction, but most of the time it is not tracking correctly. Has to be a bug.

Factorio v0.17.79
Bob's Vehicle Equipment 0.17.5

Seems to be a general problem with mimal range, same seems to happen with plasma turrets. As soon as a target enters minimal range, it seems that those weapons fire in a 180° or 120° arc from the object.
Not sure if this is only with bobs or with other min range weapons like flamethrower turret or rocket turret mod.

Setting lead_target_for_projectile_speed = 0 in laser-defense.lua for plasma equipment seems to be an acceptable workaround until a proper fix.

This still seems to be an existing issue. I wish it was fixed, the plasma cannon is so damn fun with a tankotron.

Also it seems mammoth tank comes without the heavy machine gun, making it harder to kill close enemies

Also it seems mammoth tank comes without the heavy machine gun, making it harder to kill close enemies

What Mammoth tank?