Units suddenly stop firing at Colossus

[Treos]

Hi all, had a mysterious issue recently. I walked a Galactic Colossus into my opponent's base. My opponent (white) had T2 PD and Ilshavohs, and his ally (pink) had Ilshavohs right next to white's Ilshavohs. White's units (PDs and Ilshavohs) fired at the GC until it was close, then they inexplicably stopped firing while the pink Ilshavohs kept firing.

The GC was standing on a hill, so at first I thought it could be an angle problem, but in that case it would have affected also the pink Ilshavohs... Very strange.

#11944165
Minute 50, at entrance to top left base
Fields of Isis, 2v2