I guess that in your case, you actually have a 1:1 relationship in your data between the two tables, which means that there are no duplicates in the (foreign) keys of both tables. However, all join relationships are always regarded as 1:N relationships in Celonis.
In the case of a 1:1, it cannot be determined automatically what the N-side and what the 1-side of the join is (because both is possible). Instead, the order set in the data model editor is used. The table on the left (source table) is then taken as the child table (1 side), and the one on the right (target table) is taken as the parent table (N side):
So in order to fix the issue, I would suggest to flip the tables of the foreign key in the data model editor. In your case, the EKPO table should be on the left, and the EKBE table should be on the right. After doing that, you need to do a complete reload of your data model.
Hope this helps!