[sgt101]

Even in the 1990's missiles would re-acquire targets after losing them - or after turning off their active targeting systems in order to prevent countermeasures being taken against them.

It's plausible that Ukraine was told where the ship was, launched the missiles in that direction and they acquired and targeted the ship autonomously when they got closer. Of course that's pretty risky as a gambit but the downside was a failed strike and the upside was a doomed capital ship.