faces are not missing ,it's just flipped,really easy fix, if you can use blender: just select the faces and shift+N to recalculate the normals, if not, there's probably an ez way to fix on the engine of your choice
and "often" is a big word, it's just for some items , not even 1%
and yes don't be fooled by their cute look, cause it doesn't look cute at all haha

