Should you allow jumping back and forth (in the navigation of the test)?
Yes. A fixed question order is problematic if tasks (questions) have different levels of difficulty and/or working times. With fixed navigation, you cannot allow students to answer first those questions that secure many points and/or that students also "know". When using random questions, student A might get the "easy" questions right at the beginning, while student B struggles with difficult questions at the beginning and then might not have the time to answer the supposedly easy questions at the end.