Merge transformation on regex renamed field

I have two queries from different datasources.

One one data set, there is a field, let’s call it queryUuid. On the other, there is an id _id.
I want to merge the two data sets using these two fields to join, but I can’t get it working.

I’ve tried using a Rename by Regex to rename _id to queryUuid before the Merge transformation, and visa versa, but it won’t merge them. How can I get this to work?

I’ve tried to use a hack to get this to work:

For each of the two fields I create an Add field from calculation transformation, which selects the field and + 0 onto it. This means that I get the string value of the field (which is a uuid) with a 0 appended.
I give the same alias to both the new fields, then the merge sort of works, except it doesn’t because after Merge it recalculates my new field to random values:

And a bunch of rows don’t have the other column that was in data set B. Still trying to work this out.