This isn't going to happen. This has been asked for since the beginning and they've completely ignored it. There are bugs with the Expand Stroke function that have existed since the beginning that they've done nothing to fix. I've all but given up. If Illustrator wasn't so bloated I'd go back, but since Affinity doesn't seem to pay much attention to users' concerns I have to find something else to use.
I create an object and move the rotation center to the bottom center. If I rotate using the drag handle it rotates around the bottom-center as expected. If I enter the rotation amount in the Transform panel it rotates around the center of the object.