Sometimes Blizzard uses normal materials for invisible geoemtry used for legacy collision in most vanilla WMOs. As a result, when imported the export logic regards those pieces of geometry as visible, batch type C. Possible ways to resolve: a) not importing legacy collision at all (bad and radical), b) creating a special vertex group for bugged legacy geometry and let user decide what to do with it.
Sometimes Blizzard uses normal materials for invisible geoemtry used for legacy collision in most vanilla WMOs. As a result, when imported the export logic regards those pieces of geometry as visible, batch type C. Possible ways to resolve: a) not importing legacy collision at all (bad and radical), b) creating a special vertex group for bugged legacy geometry and let user decide what to do with it.