I ran into this same problem just a few hours ago. I could un-bold the text, but couldn't change the italics in the slightest.
I resorted to adding a separate layout table above the repeating table with the same number of columns, but when I tried it again just now, I found that if you click some non-italic text, click the Format Painter button, and then click the header text, you can fix it, so please try that.