Tablespace NN5, name 'XXX/YYY', file '.\XXX\YYY.ibd' is missing! (no replies)

I have just upgraded from MySQL 5.7 to 8.0 and everything is fine except for a group of messages in the error log like this:

[MY-012351] [InnoDB] InnoDB: Tablespace NN, name 'XXX/YYY', file '.\XXX\YYY.ibd' is missing!

Table YYY actually exists, but not in database XXX. Five separate tables are reported like this, each with a different tablespace number, and none of them exists in database XXX. They actually exist in a totally separate database.

How do I get rid of these messages?