DivergentArrayError in mongoose while updating array of referenced docs
When using $elemMatch in a projection, do not use document.save(). Instead, manually update your document using Model.update(). In your case you should try
volunteer.findOneAndUpdate( { _id: ObjectId("567452bae5b25d6e6c1a0f7e"), localization: { '$elemMatch': { language: 'de' } } }, { $set: { 'localization.$.name' : "Neuer Name" } }).exec(//...});click here more details