Does/did anyone’s baby only sit up in certain situations?

My baby is 6 months old. Lately I’ve started taking her to more baby groups and library rhyme time. She can easily sit up for 10/15 minutes. I sit behind her for support if she needs it but she doesn’t.

At home she won’t sit up for long until she tries to push back to be on her back. It’s not even her toppling over, she’s actively trying to get back onto her back.

It’s only been these last couple of weeks that I’ve noticed that she will sit up with ease at baby groups because she’s engaged and staring at other babies. I was shocked because I’ve never seen her actively want to sit up before since going to these baby groups.

She has every sensory toy/item I can think of or find at home yet no interest in sitting up. 🥴 Is there anything you found useful to encourage baby to sit? Honestly she just wants to always be laid on her back. When we do tummy time, she will quite quickly roll onto her back. Hasn’t even attempted to ever roll onto her front because I think it’s the last place she wants to be. 😂 (If she can even do it, I wouldn’t know because she hasn’t tried)