Log files are opened in append mode to allow external rotation tools. When UL is configured to log to a file that already exists it doesn't truncate that file before writing to it, although this is how the previous -Xloggc system worked.

After mailing list discussions [0] it was brought up that truncation wasn't the preferred way to handle existing log files. Instead an option for file outputs in UL should be added so that users may configure how existing files should be handled.

[0]: http://mail.openjdk.java.net/pipermail/serviceability-dev/2016-January/018649.html