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I was so sure that in past versions, unchecking a field in the Join tool's embedded select would remove those fields regardless of whether I was turning it into an outer join through the union tool or not. I may be a version behind. Regardless, I see the inner join output missing those columns, but the following union picking them up from the left or right unjoin anchors.
As the left and Right outputs of the join tool are unjoined records, they will always contain all the fields from their respective inputs. The Select functionality in the Join tool will only apply to the joined records.
You would have to add a Select tool to the Left output before the Union tool to remove the fields you don't want in the output.