http://www.pcgamer.com/hidden-scaling-in-destiny-2-may-be-drastically-reducing-xp-gains/

A dedicated Destiny 2 player and redditor by the name of EnergiserX has raised some interesting questions about the way the game grants XP, and more importantly, how it may be curtailing XP gains, particularly for players who "power-grind." What's happening, according to his testing, is that the game indicates XP increasing at a normal rate using the bar on-screen, but your actual progression can occur at a much slower rate. 

EnergiserX began the testing by recording a week of gameplay, as well as periodically recording the amount of XP required to earn a bright engram as indicated by DIM, the Destiny Item Manager, which tracks and compares all sorts of in-game stats. At the end of the week, he went back and watched it all, entering every XP chain he earned into a spreadsheet, along with a timestamp and how many pixels each chain contributed to the in-game progress bar.