I was being imprecise for concision sure
TryAPL, an environment executing Dyalog (17?), when queried produces the result etc; TIO set to "Dyalog Unicode" produces the other result, the Dyalog 16/17 macOS apps when queried via the REPL produce a third result. "⍎ doesn't localise" is why I had scare quotes around "correct", and your fourth suggested result of NOP definitely makes more sense to me than the existing ones; or like, a function with optional ⍺ which in its presence returns ⍺ ⍵, and absence just ⍵