if you're struggling to unlock this achievement by it bugging try this:
Go to: C:\Users\%USERNAME%\Documents\My Games\Rocket League\TAGame\SaveData
Cut the save data to the desktop.
Turn steam into offiline mode
Start up Rocket League (You should have none of your settings etc. saved if done correctly)
Then complete a season as normal and the achievement should pop!
When you turn back online it might take a second for it to show but it will