How certain are you that the endmill is actually the advertised size?
I had some inexpensive but good endmills I got off eBay. They were name brand, but I discovered they were pretty far off in terms of diameter. I don't think they were as bad as 0.5mm, but by the time you are off on both sides of the pocket, maybe so.
I wound up measuring test cuts to see what the actual diameter was.
The other culprit to consider is tool deflection. Are you doing a very light finish pass? If you run it again does it still cut chips?