This is far from being a major bug - it can be easily worked around - but as is it probably easy to correct: it is impossible to add more than 4 basic lands of the same kind, edition and "picture" to the side board.
As an example you can't add more than 4 Zendikar forest (1) to your sideboard.
Hopefully, you can add 4 (1), 4 (2), 4 (3)...
