Round corners sticking

I have an issue with a shape, it's not got rounded corners, but displays the stroke as if it does, if I turn them up they round as expected, then drop back down as I slide the handle at the corner and as it notes it gets to zero they snap and display as rounded again. 

I've screen recoded the issue, but I'd seemingly have to host it somewhere to show here. 

I've uploaded a screen cap, you can see it is 0px above. 

Thanks in advance!