I don't know if anyone has reported this bug, but I unloaded a leader at a Union port in the Florida Keys (it was supposed to be a simple layover). The next turn he wasn't able to get back on the ship, and I have tried to load him a whole variety of ways for the last ten turns and failed (e.g., making him into a division, waiting until he was active, moving other transports there...). I have finally decided it must be a bug.
There's other times when I can't get some units in ports to combine and recombine the first time, but by moving them outside of the port things usually work fine. These types of problems always seem to be associated with units in ports.