I think it happened with recent changes to targeting system. Now, as far as i know, units will lock on whatever unit entered their weapon's range (even acu) no matter what. After that (target is destroyed or out of range) their target behavior is normal. As shown in this replay (#17336961) it is quite important for early raiding and maybe will affect ACU on ACU fights.
- Tank raiding engi near mex. Mex enters tank's range first and now thaam will focus this mex until it dies or no longer in range. Direct attack or retargeting hotkeys works to change target from mex to engie tho.
- Same story but with ACU. If ACU is exposed like this then it will be targeted until dead or exit range of unit.
Is it known behavior? Could we fix it?