Never go back to work after resigning

Two months after a colleague resigned, she wanted to return. The leader smiled and agreed, warmly welcoming her back.
At the time, the colleague left because a family member was ill and needed care. She wanted to take a one-month leave, but the leader did not approve, so she chose to resign. After her family was discharged and recovered, she started looking for jobs and interviewed with other companies. Some companies even sent her offers. However, she heard that the person replacing her was having difficulty adjusting after her departure, so she contacted her supervisor. The supervisor was very willing for her to return and applied to the department head. After approval, the supervisor informed her to come back to work, with the same salary as before.
After returning, she still worked diligently as before, but the leader always found fault and sometimes subtly reminded her to cherish what she has.
A few incidents happened in the meantime that completely disheartened her. Despite her supervisor’s earnest pleas, she still chose to resign. On her last day, she said: "Never eat the bitter fruit of going back; it will make the leader dislike you."
If she truly wanted to come back, why did the leader still nitpick?
If the leader didn’t accept her, why did he agree to her return?
What was the leader thinking?