When docking or un-docking crafts, the number of Kerbals sometimes decrements by one.

For instance, I docked two crafts together: A has 2 kerbals, B has 1.

After docking the craft only lists two kerbal portraits in to bottom-right of the screen,

There are two simple workarounds:

1. Get all kerbals out of both crafts before docking/undocking then put them all back in.
2. After docking normally, EVA one kerbal then re-enter the ship and all kerbals will be visible.

Should no longer be a problem due to the kerbal reassignment.