When using a synchronous flow step, the sub-fields for the Returns section overwrites the returns for the original flow.
Illustrated in the image below is an example of a flow with a Record Generator step as a source step with a Synchronous Flow step as a destination step.
To test the synchronous flow step, add a calculator step after the sync flow step as shown in the below image.
Before running the test, untick Use Destination Logs as highlighted in the below image. This is used for testing to see if the sync flow can run but it does not actually run the sync flow.