Define Business Key in Form Data of Start Event, but in Process Instance it is Null

I assigned a Form Data field to be used as Business Key:
But in the process instance, the Business Key is always Null, although I fill it in the start form:

I am not a thoroughbred developer but a business analyst with basic programming skill trying to build a showcase - so maybe I have missed something?
Thanks for helping here.
Ulrichstrucured-busines-vocabulary.bpmn (6.5 KB)

Hi @UGumgowski,

could you please attach your process definition file?

Cheers,
Askar

Hi !
I have added the process definition file.
Thanks and bgrds, Ulrich

Exactly the same issue here