It’s not important but in the end we want the Unreal Engine to be perfect - don’t we? 
This is not a bug but in my opinion it is confusing. In the BP editor, the “Append String” block has “Add pin+” on the output side and it adds additional inputs.
In my humble opinion this is confusing. For example, I figured it is out there completely by accident because my brain simply didn’t expect that something on the output side could affect the input.
Best regards,
Yansen.