Fixed.

I liked the behavior prior to this fix, myself. It was thematic. A lonely one on one fight in the middle of the lake. This fix could also have the effect of making the fight easier for a mindcrafter with telekinetic blast (the summons can act as a meat shield). Perhaps a more elegant solution (with the same intended effect of making the fight more difficult) would be to restore the old behavior but beef up the skeletal king a bit by making him spawn with a bit more experience.

Oh-ho, this'll make the piranha lake a lot more interesting! Though I assume he still won't be able to summon in the lake?