The spell would hit (pending a successful roll)
There is no time differential between spellcasting and 'hit' when dealing with Instantaneous casting times.
If a spellcasting has been completed, we look at the duration to understand how long the effect lasts/takes. If the spell is Instantaneous, then
The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant.
In addition, Range is no longer a factor once cast:
Once a spell is cast, its effects aren't limited by its range, unless the spell's description says otherwise.
Therefore, a spell that has completed it's casting with an instantaneous effect would deliver that effect before the target had an opportunity to move.
The only way to move out of range would have been to do so before the casting was completed