I think it's quite obvious this is some kind of bug, not necessarily bad AP count but more likely borked internal stats for combat speed.
Actually, I'm starting to suspect it may be bad AP count of sorts, because I initially climbed that tower in the upper left-hand corner of my composite screenshot and had three of my guys initiate combat with two sniper shots and an AR burst against one of the DBMs down below.
The other three DBMs responded (quite logically) by huddling against the side of the canyon to get out of LOS, and then they went into ambush mode.
My guess (and it's not a great guess, just half a suspicion) is that their unused AP aren't capped, and continue to accumulate until they decide to use them... but only for movement, it seems. They can never shoot 4-5+ times in a row, for example.