This issue has persisted since Forza Motorsport 6. The Shelby GT-350R is probably the most inaccurate Xbox One model in Forza (not counting models based off of Xbox 360 and earlier), despite only a third of its body being the issue.
In a shocking twist of events, this 2016 Shelby GT350R is the only Mustang model of this generation to have an inaccurate model to this extent. Even the 2015 Mustang GT that debuted in Motorsport 5 has a different, likely fully accurate front end.
The 2020 Shelby GT500 seems to be free from this issue with the front end as well.
In theory, a simple solution would be to get the front from the GT500 model (or the 2015 Mustang GT model), apply it on the GT350R model, and then just model a GT350R bumper for it. Really, the rest of the GT350R model is okay, it’s just the front that’s painfully inaccurate.