I updated LyX to 2.3.0-1, and found that LyX was displaying all the "-" characters in Math Mode as "\lyxminus". No such problem occurs when entering outside of Math Mode. LyX also seems to have no problem compiling to TeX, and the PDF output looks normal.
I removed and re-installed LyX, and this issue was still there.
Does anyone have any ideas on what might be going on? I am using Linux Mint 18.2 Sonya.
Note: I previously saw this question ask about the same thing, but the author appears to have deleted their post: https://tex.stackexchange.com/questions/424291/lyx-shows-lyxminus-instead-of-simple-minus-after-updating-to-2-3-0