I think that's probably it - is the AppleScript looking at all the events in a particular Calendar and getting the latest one? I have thousands of events in my Calendar.
Currently, to create new Calendar events I'm using a variation of this Macro by @Jim
It works well and ends with the simple command "show newEvent". But I couldn't adapt that AppleScript command to work to open a new event created by the native Keyboard Maestro Action.