I also had this problem (the underscore does not appear in the text).
After some research I realized that the fact is a consequence of the size of the characters, and by increasing the size of the characters, the underscore sign is clearly visible.

To solve the problem just access the view menu and then the <change font> sub-menu.
By accessing this option a dialogue appears in which at the bottom it is possible to vary the size of the characters using a slicer.

For me it was enough to set the value to 12 to allow to see the underscore sign.