>>12813230She was never actually signed to Stardom. She was always a freelancer, but before her injury she was just consistently working Stardom shows (similar to Fukigen). After getting injured and being out for a long time, she slowed down her wrestling schedule and focused more on her bar since she couldn't handle being a Stardom regular.
Ultimately, Natsu Sumire is just a fragile Yamato nadeshiko who couldn't handle getting sawdusted by Stardom's schedule.