WCSExtendlet / WPDExtendlet have a switch to disable caching for development purposes:

https://www.zkoss.org/wiki/ZK_Configuration_Reference/zk.xml/The_Library_Properties/org.zkoss.zk.WPD.cache
https://www.zkoss.org/wiki/ZK_Configuration_Reference/zk.xml/The_Library_Properties/org.zkoss.zk.WCS.cache

The ZumlExtendlet currently doesn't have a switch to disable caching in a similar way.

I suggest adding a new option org.zkoss.zk.ZUML.cache and disable caching accordingly.

Workaround

reduce the cache check period (which currently defaults to 1h)
https://github.com/zkoss/zk/blob/v8.5.0/zk/src/org/zkoss/zk/ui/http/ZumlExtendlet.java#L80-L85

<library-property>
	<name>org.zkoss.util.resource.extendlet.checkPeriod</name>
	<value>5</value>
</library-property>