You need to put formatted content in using the formatting tools at the top of the input area on this forum. The backslash is reserved for something when used in-line, I believe. It's not a bug, it's a feature ;-)

Update. There is definitely a problem with the display of the symbol for [Rule] in ie11. When I enter an option in an expression literally with [Rule], the display turns it into a blank, but the expression executes correctly. So, the symbol is "there", just not being visibly displayed. Trying to reformat the cell with different fonts or font sizes doesn't help.

ha!, even in this post there is a problem. My original post shows a "left backslash", a "bracket", and then the word "Rule". When the post is published the "left backslash" character is missing.