Baby sits with legs stiff and straight

My baby is 11.5 months and generally has been hitting all milestones (though not crawling yet, but rocking on all fours and army crawling).

However for a while, when sitting usually sits with legs stretched straight out and stiff to where it almost rocks her backwards and her ankles/feet come off the floor and kind of rounds her spine. She’s been sitting independently since a little before 6 months so is excellent at that. But her rounded spine is bc she won’t really relax her legs to sit up straight. It’s especially pronounced when she’s sitting and gets excited about something. She almost stiffens them even more and waves her arms around or claps and her feet come off the floor.

I’ve mentioned to her pediatrician a couple times and he even saw it while she was sitting on his exam table and he seemed to brush it off and wasn’t concerned but I’ve not seen any other baby sit like this or do that (she’s around a lot of babies at her childcare, and our moms group playdates etc). And I can’t really seem to find anything about this online.

Has anyone else’s baby done this? It’s been this way for a couple months. She’s otherwise a super happy, smart and social baby it’s just this very noticeably odd posture.

Thanks in advance for helping out a worried first time parent.