I get that from time to time (and you and I are not the only ones). I don’t really understand the circumstances that lead to it. However there have been other discussions e.g. "apt upgrade" says "The following packages have been kept back" so a forum search may be in order.
I would always do sudo apt update if I received the ‘kept back’ message, just in order to make sure that I was up to date with the package metadata.