I have the same frustrations with losing painted vegetation when I reload my terrain surface (FBX export from Autodesk Infraworks). We've been hoping to use Twinmotion to support visuals throughout the entire design process...meaning our terrain surfaces are getting revised constantly, also meaning we're constantly losing our painted vegetation. Perhaps instead of us waiting on a rewritten importer, maybe Twinmotion should consider the addition of a special model category or tag of sorts that be assigned to any imported object(s) we might want to use as a terrain for hosting painted vegetation. Then revisit the "Painted vegetation" tool by having the ability to assign/reassign the existing painted vegetation object to any selected element(s) you've designated as a Terrain....thus snapping the existing painted vegetation to the newly imported surface you've designated as "terrain". Whatever the fix ends up being, we need Epic to to resolve this issue sooner rather than later, as it is a constant loss of work for many of your users in the Design community.