Thanks a lot @pitu72 , this solved my problem.
I am now sending the variables transient via the REST-API to the event-subprocess:
{
"messageName": "demoMessage",
"processVariables": {
"localVariable": {
"value": "exampleValue",
"type": "String",
"valueInfo" : { "transient" : true }
}
},
"resultEnabled": true
}
And then save the variable at message start event locally via execution listener: