My girl does this too when she’s 4mo old. Able to roll on tummy and lift head, but unable to roll back . There was once I wake up to her sleeping on her tummy, but she’s sleeping on her side face (ie: cheek) & i was relieved. I too wait for her to fall asleep before I go to slp to ensure she fall asleep on her back instead of tummy. (Ie: flip her to back whenever she turn on tummy) . So I can go to slp without worrying