In some cases (I only found numbered equations), the LaTeX output still has a bit of HTML. For numbered equations, using \tag{X} (as suggested in https://www.lyx.org/trac/ticket/13048) would require fiddling with the output. KaTeX doesn't support \label. This patch is probably the easiest and most reliable fix.
Fixes https://www.lyx.org/trac/ticket/13048.
