I have bad news for you - V-Ray has the same problems. So this is not Corona's issue.
Bercon is too buggy :( For example, try to use it in bump slot with textures in tiles. You need to learn some kind of dark magic to use this plugin.
Textures are not missing, they became blurred when you load new maps in MultiTexture. Maybe it's MultiTexture bug and it doesn't update filtering settings after loading maps.