I am not working on this right now, I am just pushing things I tried a couple of days ago.
No one complained yet and I think it's not so super important right now because some edge cases are really total edge cases.

SeeFixJavaHelpers.updateRequestWithUri #12977 (comment)

Like said in that comment we use a different logic to create the query map and in some cases the methods behave slightly different. Of course it would be better if all methods use the same logic/method.

Test code for sample app to test locally

Java action:

publicResultindex(Http.Requestrequest) {System.out.println();System.out.println("one:" +request.queryString("one"));System.out.println("two:" +request.queryString("two"));System.out.println("three:" +request.queryString("three"));System.out.println("empty:" +request.queryString(""));returnok("one:" +request.queryString("one") +"<br>" +"one?+" +request.queryString("one?+") +"<br>" +"two:" +request.queryString("two") +"<br>" +"two=:" +request.queryString("two=") +"<br>" +"three:" +request.queryString("three") +"<br>" +"empty:" +request.queryString("") +"<br>" +"empty_all:" +Arrays.stream(request.queryString().getOrDefault("",newString[]{"<empty>"})).collect(Collectors.joining(","))        ).as("text/html");

Equivalent Scala action:

defindex()=Action {implicitrequest:Request[AnyContent]=>    println()    println("one:"+ request.getQueryString("one"))    println("two:"+ request.getQueryString("two"))    println("three:"+ request.getQueryString("three"))    println("empty:"+ request.getQueryString(""))Ok("one:"+ request.getQueryString("one")+"<br>"+"one?+:"+ request.getQueryString("one?+")+"<br>"+"two:"+ request.getQueryString("two")+"<br>"+"two=:"+ request.getQueryString("two=")+"<br>"+"three:"+ request.getQueryString("three")+"<br>"+"empty:"+ request.getQueryString("")+"<br>"+"empty_all:"+ request.queryString.get("").map(_.mkString(","))    ).as("text/html");  }

Testing with:

http://localhost:9000/?one=&two=a+b&http://localhost:9000/?one&two=a+b&http://localhost:9000/?one=&two=a+b&=http://localhost:9000/?one&two=a+bhttp://localhost:9000/?one?%2B=&two%3D