Very interesting. I have these same shoes but in white. I've never had the issue you describe but then again I've never tightened them to the tightest setting. I'm curious, what are the instructions that Giro has for releasing an over-tightened buckle?
Though I've never had this issue with these shoes, I can say they are not my favorite shoes by any means. I always found the fit to be a little off, despite trying different sizes, resulting in a bit of annoying heel slip.
I think, since I rarely wear these shoes and would love to have a justifiable reason to replace them, I'll go try to over-tighten them and see if my buckle gets stuck.