Firstly, the field is not shown by default in the Metadata panel. You have to go to the Metadata tab, disclose the EXIF panel at the bottom and then select the checkbox beside the Lens Model field.
Secondly, this information is only extracted if you are using the RAW 2.0 decoder. If you import images from an older project using RAW 1.1, the field will be empty. Similarly, if you migrate an image back to RAW 1.1, the contents of the field will be removed.
Finally a little trick: you can separate your zoom lens shots from your prime lens shots by searching on whether the Lens Model field contains the "-" character or not.