I see this often, some units no matter the race, do not make their ability (!very important! - without being under enemy attack or enemy stun or other abilities) and attack through regular attack instead, wich makes the player to push these abilities zuking times to actually use it, and it is frustrating, when you have to control many units and some of them just dont do they ordered to and cause important moments loss.
I saw this on scorpions, angelos and etc, but if devs are going to test it, the best thing this glitch or thing happens is assault terminators, just try to order them to missile barrage on targeted position (better if its position is out of their sight, because when target in sight, this glitch sometimes happens, sometimes not and actually most of times they do barrage the target), if there any enemies in their circle sight, they cancel barrage ability on target position and just attack those whom they see.
More simple like that:
Target is out of sight - order to barrage on target position - terminator with rocket pack start to moves so he can make barrage - if any enemy unit is in sight cancel order and regular attack on closest one.
Dont test it if enemy units are not in sight, if none spotted they just normally moves and use their ability.
This glitch may happens on various, both regular and elites units, without being in melee, stunned or anything.
I can record and upload a video on youtube if you need a more complete report of this trouble.