Hi @Simon_Turvey,
Apologies for the confusions on this issue, very weird behavior as it looks like you set up the child workflow correctly and it should bubble up the data to the top level workflow.
This may be a bug, I tried to reproduce this but the child workflow was able to return to the top level. But if you were able to reproduce this then we definitely need to investigate further and hopefully it is a small 'gotcha' issue.
Would you be able to attend our office hours? We can look at your app with a team of support engineers to get lots of eyes on the workflows and test out your reproduced simple workflow as well.
We are office hours every Tuesday and Thursday at 11 am PST. Check out the link here for how to join.