I've also noticed oftne times when I'm Advent and I convert enemy sihps, they'll be in their own fleet and for some reason fly off to other planets without me telling them to
Since those converted ships were apart of the enemy fleet, they actualy keep there own fleet and continue on there last movement orders from your enemy, That is a pain, so I think the devs should fix that in a new patch, however most people have grown use to it are able to manage it.
was maybe 150 ships
There you go thats the problem, all the devs know of this issue, since a lot of the logistics are used up, for some some reason a certain few ships get lost on an occasion. The devs are trying to fix but can't figure whats causing or why in world a ship half leaves your fleet orders. This happens to me espicaly in the late game with all logistic points spend on ships.