The key thing here I think is the replacement by the starter. That's allowed but the same starter cannot be replaced by the same sub. The sub can re-enter for anyone else but the starter that replaced her.
ISF rules
An illegal re-entry occurs when:
c. A substitute who has legally been in the...
Ok, in a game earlier this season, basically the same thing happened and the batter was called out. She entered as a pinch hitter and was removed for the starter. Entered as a hitter again, got a hit and was called out for illegal re-entry
in a game recently, the opposing team had a sub hit for the starting player (who was a player on the field, not a DP/Flex) then removed the sub to re-enter the original starting player. A couple of innings later, the same spot comes up, the coach removed the starting player again and...