Per the description, I am finding that when I start to build a morophon commander deck and add mountains, it adds to the overall card count but does not show up in the deck list. If I were to grab the whole deck and move it to the sideboard it will all move EXCEPT for the mountains which will now appear in the deck list as Morophon is no longer listed as the commander. If I were to change the commander or deselect morophon as the commander the mountains return.

Thanks for reporting the issue. Fixed today.