Help me revert my "bold" underline back to normal

Hi everyone.

Okay so to clarify, I took a screenshot to explain what I mean by my "bold" underlines:

QRG5UUl.png

Notice the difference between the bold underline and the normal ones. There is also some weird thing happening where in some cells, when I use the underline feature, it shows as a bold underline, but once I've pressed enter to move on to another cell, the underline reverts back to normal. If I select to edit (F2 or double-clicking) said cell, the underline becomes bold, and vice versa (s3lw.png).

All this started happening immediately after I did this (was just experimenting, and I have no idea what this actually does, or if it was the cause of those occurences): Insert > Symbol > Special Characters > No-width optional break. I did that to just one cell though, and now all this weird stuff is happening. Help me fix this please!