It's getting encoded. % is a special character in URIs. Let me try posting it inside of back ticks, as well as triple back ticks:

https://noai.duckduckgo.com/?q=%s

https://noai.duckduckgo.com/?q=%s

I've noticed that there's a plague of threadiverse clients which improperly escape/encode URIs. It's most evident with how they mangle parenthesis in Wikipedia article titles.