It looks like every attribute is having it's value [re]set, rather than just the one that you changed.
Whether this is a problem in jmx-console, or JettyMBeans need to ignore having current values reset on them needs to be investigated.
I will put it on my list.
OK, I've tried this.
It looks as if the jmx-console code is not bothering to check which items you have changed, but is simply resetting every writeable with the value held in the page.
Some of Jetty components have attributes that are only writeable before the component is start()-ed. Hence the warnings.
I would prefer that jmx-console cached and compared the values that it was using, but it looks as if Jetty is deciding correctly what to do.
I guess you will just have to live with the warnings, now that you know why they are being given, or persuade whoever writes the jmx-console app to set attributes a little more sparingly.
If this actually breaks any Jetty behaviour, let me know and I will look into it.