@Wouter_Nigrini I managed to fixed the issue. 
I remembered this post I read yesterday. The problem described in this post, is quite similar to mine. One route of the workflow wasnât returning data. No idea why I didnât get the deeper meaning of it earlier, but yeah, maybe it was already too late yesterday 
The issue was, that the workflow didnât return the data, as the MERGE node on another route didnât get data for all inputs. So this node was still waiting for incoming data and didnât finish, even the workflow itself finished successfully. Seems like this leads to some strange behaviour with finishing a sub-workflow and returning data.
So, the problem in my workflow was, that there is an if node, that checks if the so-called corekey exists, before further checks are carried out. For the âinvalid corekeyâ route, the merge node is not getting itâs second input, as the if node activated the other route and so the merge node was waiting for the second input. (see screenshot).
The purpose of the merge node is, to merge the data from the input data and the result from the SQL query (maybe there is a better solution than this). The solution to my issue was, to change the positions of the merge and the if node in the flow. So, merge both data paths first and then decide, what route to activate.
So, even if the workflow was working successfully, it was logically not fully correct.
Thanks for your time and help.