I am not sure it is a bug. I think percentage of ships in fleets means maximum percentage. Because the fleet minister breaks up fleets quite frequently numbers like 80 or 90 will never be reached. Have you tested with low values like 50 or even 10 or 20?
Of course the minister could need some improvements
. Like not breaking up a fleet after a planet is captured or looking for repair bays in the fleet rather than breaking up for repairs.
Rollo