When the istat daemon emits log messages only a partial header is prepended. This means that in systems with many databases it may be impossible to tell which database the message belongs to.
A workaround is to enable tracing, but that results in increased log volume. Most of the additional information is of no use during normal operation.
Another way to identify the database is to look at the table names or table identifiers.
The correct fix is probably to include the database in the log message header, which is what's done for many/most other messages in derby.log.