Sounds like you might have a set field script step that is setting data into a related field from the child table which is causing the new record creation.
Did I notice that your relationship uses the Child table on both sides?
So when you enter the criteria to control what is listed in your portal, you are entering data into a global record in your Child table. This is probably enough to generate a new record in the child table.
Change your relationship to use a global in your Parent table and then base the layout on the Parent table.
I think this will solve the creation of the spurious Child record.
Best wishes - Alan Stirling, London UK.
That solved, it Alan; thanks to you and rgordon, who was on the same track. Funny how this never happened in other solutions based on the same structure.
Very grateful to you both!