Slitherine

Here's and example, the US 5-strength unit moved out of the primary corner objective.
Maybe the AI was seeing an opportunity to cut the tank supply.
The US unit was most probably set on AI defense, maybe static. Hard to remember the set-up when you're in-game.

I've also noticed this once or twice in the Marines and now in the China campaign so far. The AI seems very eager to flank, cut-off, and kill something.

Usually I'm all in favour of the AI outflanking and cutting supply, but it should not leave a primary objective to do so.
I'm not too fond of the relentless attacking with often low-strength, low-efficiency units.
I would even be willing to allow the AI to cheat a bit, determine the actual combat result, then decide if it will still attack (not if the AI unit would get the worst of it of course).

IIRC it shouldn't leave when set to static def. It can be forced to retreat in that mode tho. If it came out to attack I'd assume it to be on "defend hex" with a certain range set (in which it would attack).

We can probably change it so it NEVER leaves a primary VP hex, but in this case the AI must've realised it would die the next turn anyway - so the unit chose to mess with your tank in its last possible action. The AI isn't aware of a scenario's specific victory conditions, so the real importance of a Primary VP may differ according to the scenario.

If the unit was on static defense it would never do this (there is no code for movement in that AI task). So that's the best task to use for units on Primary VPs which directly affect the outcome of a scenario.