As long as a group of coastal tiles is less than 21 tiles, and is enclosed by land, then it's a lake.
Look at this diagram:
L = Land
C = Coast
LCL
CCC
LCL
That Coast in the center is a lake, since the land connects on the diagonals.
CLC
LCL
CLC
That isn't a lake, since the islands are 1 tile too far from each other.
This isn't a bug BTW.