Migrating a subdirectory site reverts to internal domain & loses alias

As per #3036890: Simplify subdir site installation, we should be able to migrate subdirectory sites (e.g. example.com/site1) and maintain the user-friendliness of the subdirectory alias being presented as the site name. However, this isn't currently working.

After a migration completes, the resulting site name is the internal name (e.g. site.example.com), and the alias is lost.

Workaround

• Edit the site.
• Re-add the alias.
• Save the form.
• Edit the site again.
• Set all redirects to go to the just-added subdirectory alias.

All will be as it was before.