This is a bit strange but LyX installed on my system which has now MacOS Mojave has stopped showing boldface letters. It was working perfectly on MacOS Sierra. Initially, I had LyX 2.3.0, I tried upgrading it to 2.3.1 but that didn't work either. I tried searching it on Google but found no relevant information. Does anyone have any idea of what's going on?
Attempting to answer the question of "Does anyone have any idea of what's going on?", this is a bug that is specific to using LyX with Mojave, and is tracked at https://www.lyx.org/trac/ticket/11271. This does not appear to be a LyX bug. You can follow an upstream Qt bug report here: https://bugreports.qt.io/browse/QTBUG-69955.
As for when LyX could include a fix, first Qt would need to provide a fix in a new version of Qt. Then, LyX would need to use at least that version of Qt to build a new LyX version. If you are interested in building both Qt and then LyX, write to email@example.com and we can try to help you. If you don't have experience with this type of thing, it might take a while to get set up.