IntelliJ IDEA doesn't recognize certain generated source files. Specifically,

1. Postgres-related code generated from Protobuf (e.g., used by io.debezium.connector.postgresql.connection.pgproto.PgProtoMessageDecoder)
2. Oracle-related code generated from Protobuf (e.g., used by io.debezium.connector.oracle.olr.client.OlrNetworkClient)
3. DDL parsers generated from ANTLR grammar (e.g. for MySQL)

To fix that, the corresponding paths could be manually added as source paths for the corresponding modules. Alternatively, a Maven plugin (e.g. build-helper-maven-plugin) could be used to add them as sources automatically.