I'm a developer who's just inherited a bunch of Open Cart sites. Whilst I'm new to OC, I'm not new to web development so in a sense know what I'm doing

Have come across a weird issue with some orders on one site where we're missing information; for example last name, or postcode. The only correlation I can find between these orders is that they use the Paypal, or Paypal express checkout module.
My question is, how does the Paypal module work? Does it use information from Paypal to populate the billing/shipping address? Or does it use the OC checkout form? Also has anyone else experienced this issue and know of a solution?