Thanks for the response Paul.

I gave your solution a try but unfortunately it didn't work. Just to be clear, the first attachment is the cell containing the problematic bold text (as well as the non-bold text in the last line). The second attachment is the same text as it looks when I've selected the text, as you do for something like copying and pasting. I thought it might be useful to show that the shaded part for the bold text is a little bit darker than the shaded part for the non-bold last line.

It might be hard to see in the attachments, and I probably should have explained that better. I originally noticed the shading difference because I highlighted the text thinking I could just deselect the bold button and resolve the issue.