Potty trained but refuses to 💩.

Anyone have any experience with a child that refuses to poop in the toilet. She is otherwise potty trained but has a lot of anxiety (?) around pooping. She stands to poop and I’m not sure if that is part of her issue- she never squats. This has been going on for many months and isnt a new issue. We are keep it positive and no pressure or punishment. I found her today ready to poop and she walked to the potty happily but hours later is still holding it. We can “catch her” in the act and she holds it. I have no idea how she manages that! We see the dr for her 3 yo check up in a month so will look for advice from her too but was wondering if anyone has experience. Thanks!