Dataflow with different data sources fails

I have a dataflow where I would like to combine 2 tables from 2 different datasources with a merge query (merge as new), however I keep getting errors with the connections.

Here is the situation:

• Table A: Oracle on premise database (using on prem gateway)
• Table B: Dataverse table

Both tables share a same column where I would like to execute a merge query (join left-anti) into a new table (Table C).

This is the result:

If I merge the other way around I get this:

I have the "Allow combining data from multiple datasources....." checked, but this doesn't change anything.

Is it simply not possible to do this or am I missing something?

Thanks!