Negative amount of remaining transactions before wraparound
That just means that your query is bogus.

age(xid) compares the age to the current transaction ID, so it is useless to call it with a multixact ID.

Moreover, running that query will return nonsense for anything except for regular tables (relkind = 'r').

You could try something like

SELECT cc.next_multixact_id::text::bigint - t.relminmxid::text::bigint
FROM pg_class AS t
   CROSS JOIN pg_control_checkpoint() AS cc
WHERE t.relkind = 'r';


to see how old the oldest multixact is.