Hi Jonathan!
Can the height difference between two vertex loops cause strange results?
https://i.imgur.com/OfAdsag.png
The first picture is what I got the first time I applied the subsurf modifier. After that, what I did to correct those shapes was deleting some vertex loops and extruding new loops from the closest ones. I don't know if the height was really the problem - on those faces circled by blue you can see that there's a vertex not on the same height as the others but the shape is not affected, making instead a concave edge - but this is my best guess at the moment.
Can you tell me anything that can cause the same strange results?
Thank you so much! (and sorry if my question seems a little confuse. English is not my first language)