Before today, ClickHouse users would only see the tables in the default database when querying table metadata from ClickHouse system tables such as system.tables or system.columns.
Since users already have implicit access to underlying tables in r0, we made a change at 11:05 to make this access explicit, so that users can see the metadata of these tables as well.
I'm no expert, but this feels like something you'd need to ponder very carefully before deploying. You're basically changing the result of all queries to your db. I'm not working in there, but I'm sure in plenty places if the codebase there's a bunch of query this and pick column 5 from the result.