The FastRollingFileManager used by RollingFastFileAppender extends the core RollingFileManager.
I made a mistake in my implementation of FastRollingFileManager and it does not correctly create a new RandomFile during rollover.
To fix the issue I need a small change to core.appender.rolling.RollingFileManager#checkRollover(LogEvent):
inside this method there are two lines where the new FileOutputStream is created and passed to the parent OutputStreamAppender.
If these two lines can be extracted into a protected method, subclasses like FastRollingFileManager can override and implement their own behaviour.
Patch to follow soon.