Althought this bug I detected in Twilight 1.95, but I didn't see it fixed in 1.96 changelog.
Example (Acean Empire which I played):
I have A-class ship and B-class ship.
A-class has speed of 12 pc/week (traded tech, got Warp drive III)
B-class has speed of 6 bc/week.
A-class moved, 0 pc left. B-class moved into A-class square, also 0 pc left. Creat A-class +B-class Fleet. -> disband.
=>A-class remains 6 pc, movable...