Hey,
As I always do some map exploration before finish the main campaign, the idea is get a better overview of the story, I notice that after reach level 40 all my leveling ups before finish the campaign don't reward anything and this should be wrong.
Take a look at this video where I got 2 leveling up in 2 minutes and don't win a sh*t.
https://youtu.be/uuHnN5cQG1Q