Hello Timo,
Seems like a bug on Line 116 in Example_06_DataStream_Join.java, where PerformJoin is called when no transactions exist. On the contrary it should be the other way around i.e. PerformJoin should be called when Transactions exist for a given Customer. Hope this makes sense.
Reference File
https://github.com/twalthr/flink-api-examples/blob/main/src/main/java/com/twalthr/flink/examples/Example_06_DataStream_Join.java
Warm Regards,
Nilesh Padbidri