did you use bias? it's stretchy, which really doesn't help at corners.
most likely, it's because of all the bulk that piles up at corners. when you turn everything right side out, all the stuff has to go somewhere. (like when you squeeze beeeeeeeeeeeg hooters into a too-small slingshot. :lol: :lol: :lol: )
i have tried most of the methods i've found on the internet for mitring binding at corners. don't like any of them because i always end up with the pointy-lumpy things, too.
i mitre the corners of my binding with seams now, as though they were borders. when i have instructions written to go along with the photos i took the last time i did a binding, i'll share. i doubt seriously that my method is unique. it's bound to be out there somewhere already. [don'tcha love that pun? :lol: :lol: :lol: ) i just don't know where.