– It is unintentional for DuckDuckGo. The code for DuckDuckGo works correctly but no one who wrote that code thought about whether a reversal would happen.
– It is intentional for the browser. The code for the browser works correctly and someone who wrote that code actively thought about how to make a reversal happen.
I don’t think ‘accidental’ is the right word to use in either case because the outcome is what you would want.
The reason I used "accidental" is because it's not a bug (and you've alluded to that same conclusion too). You could argue it's accidental from the perspective of DDG if it happened by chance rather than design. But the distinction between "accidental" and "unintentional" is nuanced and I'd already offered "intentional" the alternative option so I'd argue you can pretty much use them interchangeably in this specific situation.
– It is unintentional for DuckDuckGo. The code for DuckDuckGo works correctly but no one who wrote that code thought about whether a reversal would happen.
– It is intentional for the browser. The code for the browser works correctly and someone who wrote that code actively thought about how to make a reversal happen.
I don’t think ‘accidental’ is the right word to use in either case because the outcome is what you would want.