I don't think it's a bug. Some of my custom foods, sausages for instance, are displayed with a serving size of "1 link." I have other custom foods that are displayed with serving sizes measured in ounces, cups, etc. And I have some that are displayed in metric measurements of fewer than 100 grams.

Have you checked your settings (click "Edit Settings" on the menu on the left of the page when you're in My P.L.A.N.)? You might have it set to display in 100 gram increments. I have mine set for ounces.