I'd agree here you cannot perform a shoot action on a model that is "adjacent". Even if you were not "pinned" by a fight action. Unlike spells which can be cast without the need of breakaway/free strike.
The better way to say this is simply
to get this bonus Madriga's arrow needs to pass through one of the squares in the rear arc Of the model being shot at via an unobstructed LoS.
If you were to shoot from behind on an opponent where your LoS had a table in the way then you'd both face a -1 dice penalty.
This is why facing your guys before ending that pieces turn is so important BTW.
Sadly it seemed like some rules were thrown out there with seemingly the assumption that "you know video games work so you should be able to get what we are thinking", rather that having a good explanation/practical example like twighlight was able to do.