Underlined text/mtext issue

I'm having an issue with underlined, stacked text within mtext objects. A plot preview (see PREVIEW, attached) shows the underline as plotting correctly. After plotting however, the underline shows up as very thin or shaded (see PDF PLOT, attached). We have also run into this issue with text objects using the %%P tag to underline them. In the latter case, we convert the text to mtext and underline it using the button in the mtext editor, and it works fine.

This is one of several text/mtext issues we have had, most of which we've fixed by recreating the objects in our templates, but in this case, recreation doesn't do anything to solve the issue.