The Athletic has an article on him and says that he didn't work very hard at being good. Dropped a lot of balls. He admits to it and that even after signing with the Bills he wasn't working hard enough.
Getting cut earlier in the season apparently lit a fire in him. He went all in and spent his free time working to get better, studying, practicing on his own, etc. His roommate and friend, Bills player Keith Ford said,
“I’ve seen him work ever since the beginning. I already knew. He was miserable, but I can tell you he put his head down. I’ve never seen him grind so hard, because he wanted to play.”
https://theathletic.com/717759/2018/12/16/that-boy-is-balling-robert-fosters-historic-production-sparks-bills-win-inspires-fellow-rookies/