Help... 3 year old won't poo in the toilet

Please help... My 3 and a half year old daughter has been toilet trained since she was 2. But she will not do poo on the toilet. She has now built up anxiety about it and I am out of ideas how to help.
She likes to do it standing up in a quiet area on her own and will often go and hide to do it in her underwear.
She has done it before but I think it's frightened her and she won't do it again. I think putting her back into pull-ups will just be a backwards step and confuse her more.
We have done rewards, watched movies, read books about the toilet, given huge praise lots of incentives but she is just not interested in any of it. She will happily sit on the toilet got long periods of time but won't do anything but will do it shortly after. I've given her space on her own and a quiet place to try but nothing.
I've got the toilet seat with a step and a potty.
Oh and she is definitely not constipated but she can certainly hold it in for a period of time.
Any ideas would be greatly appreciated as I'm all out of ideas and have no idea what to do next. She is going to kinder next year so this needs to stop!
She has 2 older siblings that never had any issues like this I just don't know what else to do??