Can somebody please explain to me what exactly happens when I import the custom json file that is already in the game. I converted the 2020rosters file to an csv file and had a look and those ratings look fine to me. converted it back and imported it and the ratings are not the same. why does this happen? can I stop this from happening? Thanks.

Are you importing them, or creating a league with default rosters? If you import it should use the ratings as-is... if you create a league with default rosters it doesn't know you've modded that file and applies a bunch of adjustments to bring that file in line with players that the game expects.

Yea I'm selected custom user-created rosters. Then selecting the 2020rosters json file. Then once the game loads up the overall player ratings are not the same as the ratings in the file. for example certain players have an 100 overall grade and fullbacks are all rated really high. How do I stop this? is there a setting i need to change/adjust when importing a roster?

If you create a league and use "Default Rosters" it will use the players.json that comes with the game in the program files directory. It will also run those players through some adjustments since the .json file doesn't lineup perfectly with expected values.

If you've copied that file off and are using it as a "Custom Roster" file then it will create players using those exact numbers with no adjusting. It uses the individual attributes ratings, not the OVR in the file... the OVR in the .json file isn't used for anything.