This should work and I just tested it to verify.
I have one thought:
When you have a flex with multiple board outlines, Xpedition maintains an internal common board outline that is the mergee of all individual board outlines. -It's illegal to attempt to change that outline -its updated automatically by Xpedition.
I think that at least in some versions of Xpediton, that common outline can be selected -But you cannot edit it.
My best hint is to use the tab key to cycle through selections to make sure that you have the correct board outline selected -then it should work.
The image below is from a flex rigid design with 5 different board outlines each with its own stackup and I can change any corner to an arc using the approach I described.
.....and if its still a problem, Please call help desk so they can give you a hand and maybe try it on your design.-it should absolutely be possible to do what you want. /Per