This not a bug.
When we add a new vehicle or plane, the "skins.res" file has to be updated. This file contains all the references to skins in the game. When it has to be updated the current version is invalid and so the game does not know about any skins installed.
Once the new version of the "skins.res" file is installed (only available online), then all the skins will show up again.
I also had to fix one of the tiger skins, which would invalidate the "skins.res" file as well.