New migrations: trim title, trim slug, set sort title if missing, and convert any ObjectIDs in _id to plain strings. A2 uses plain strings for _id to simplify JSON serialization, among other reasons. Convert pages created by folks unaware of this rule by removing and reinserting them. It's an easy mistake especially when writing an importer from another CMS. When writing an importer the best way to create IDs is often to base them on the IDs in the old system, with a suitable prefix.