Any land unit embarked (past turns in the mid-90's) get an extra tile of movement. The first movement hex isn't counted against movement (not this is just for land units, not for ships of any kind - they track regularly). 

PC Specs:

GTX 980 Ti

16GB Ram

i7-6800K CPU

DX12

Note, if you move one tile (and it doesn't count against the movement) but try to go on land with 3/3 movement not through a harbor, it will not work and require you to wait the turn. 

Reproducibility: 5/5 from when it started happening (every turn but I don't remember the exact turn it started and the circumstances)

See pictures indicating the same turn, showing starting position and ending position. with 3/3 movement.

Also has this bug. In my game, it began after research technology with transpor ship for ocena tiles.