Detective Conan said that the dim sum disappeared in the old shop and the invitation with code were the link stories of M 17 and M 18 respectively. Which link?

It is similar to the previous theatrical special edition, and has little to do with the content of the theatrical edition. Generally speaking, it is a theatrical story before or two or three days after the incident. But it will have something to do with the theatrical version. For example, time-honored dim sum takes place in Kyoto just like M 17, and this episode of Secret Invitation also has the scene of children setting off fireworks together in M 18.