The problem only seems to occur if the Geneva font is used, Chicago, Courier, and Helvetica do not have this problem. This has been reported to engineering. There are three workarounds you can use instead.
Workarounds- Use any other font except Geneva.
- Select the underscore characters (__), and change only that selection to Chicago, Courier, or Helvetica.
- Use spaces or tabs in place of the underscore characters (__), then select the option to underline.
Note: If you want even underscore lines, you should use tabs. With the exception of the Monaco and Courier typefaces, which are monospace fonts, the typefaces in your Fonts folder are proportional. This means that not all letters take up the same amount of space. If you use a typeface such as Times, underscores that look aligned when printed, will not always be aligned.