I am not sure if this is intended behavior or a bug. When I do a merge, field used as the join key in the right side does not show in the end result of the merge. Please see attached file. I am selecting All fields, but it is not bringing the field "Key". The merge is showing 22 fields instead of showing 2 fields. We can work around this issue, but just thought i bring it up to your attention.
The Key field is not preserved as its values are identical to the ones in the Isin field, no data is actually lost. The name of the column with matching values is that of the first on the merge block (the one plugged on the left).
The merge/join is showing 21 fields, this is the correct behaviour, a join appends the columns of both tables. If you want to exclude certain fields from the join you can use the "Select fields to include" in the merge block.