How did you mirror it? I saw you used an empty but what happened after that is too fast to catch. I asked this once with the previus site format and got my answer but it didnt make the migration.

That part is a little tricky. The problem is that simply scaling in negatively in X doesn't give you the mirror result that's expected. But if you take all the desired objects for mirroring, parent them to another object (like an empty), and scale that empty -1 in X then your objects will be 'mirrored' correctly.

Then you can unparent the objects from the empty using the "keep transforms" option and delete the empty.