timeCycle of timer start event not saved to bpmn20 XML file

When starting a process with an timer start event it is not possible to set the Time cycle property in Main config so it gets saved to the process definition XML. Instead an empty timeDate tag is written to the timerEventDefinition.

However, if the timerCycle value is written using an text or XML editor then the value is displayed in the Designer property editor. But once the process definition is safed again, all timer properties are lost again...