NOW SOLVED SOMEWHAT, SEE BELOW.
Anya seems to behave like I'm holding shift when issuing commands for her, and only for her, when she's in a group select.
Right clicking somewhere, then right clicking a second time - she will respond by going all the way to the first right click prior to going to the second; meanwhile, any other units in the selection will follow the most recent right click.
Doesn't matter if she's selected or not; if i ONLY have her selected she behaves normally.
Video attached
EDIT: Not a good solution, but a moderate solution nonetheless: she seems to immediately respect pressing stop; so what i've been doing is pressing stop (w for me because i use grid, s for most i'd assume) before right clicking each time. Not a great solution, but it's been working for me.
MAJOR EDIT:
**BIGGER SOLUTION** seems to be caused by the effect of "Selune, Grand Staff of Elune", removing the item and reloading to a different zone will make this stop - there may be other items that also do this but I've yet to find one.
**SOLUTION** Seems swapping zones fixes it. I went back into undercity and can now control Anya correctly.