The reason why Mai Tian knew that Zhang Yuan had sprained her ankle was because he had unintentionally heard Zhang Yuan's assistant making a call. Since Zhang Yuan was unwilling to reveal this matter to the public, Mai Tian had no intention of exposing her. However, he still decided to take care of her in secret.
Zhang Yuan was not a stupid person to have been in this show for so long. Hence, she guessed that Mai Tian must have known that she was injured. After thinking for a while, she decided to secretly thank Mai Tian, the considerate big brother.
All of this was a private conversation between Mai Tian and Zhang Yuan, so no one else knew.
Hence, Su Lan suspected that Mai Tian was deliberately ostracizing and targeting an outsider like her when she saw that Mai Tian had given the simpler second segment to Zhang Yuan. She was already paranoid about Mai Tian deliberately snatching the simple mission just to take care of his own people.