Rosegarden does not reallocate plugin processing buffers etc if the JACK buffer size changes.
This hasn't been much of a problem in the past, but now that Ardour has an option to change the buffer size from a menu option right there on the main menu bar, there's plenty of potential for havoc.
Diff:
Closing this as no one has complained in 10+ years. Also, it appears as if Ardour now hides this setting deep in its configuration stuff. Anyone tweaking this should know that a restart of everything is wise. Please re-open if needed.