Tick label can not display normally

I change the font of ticks supported by GR: Fonts — GR Framework 0.71.8 documentation. Since the default backend for plots package is GR, I think I can use these fonts. However, when I save the figure as pdf file, some symbols can not display normally, for example, the math symbol “-”. This my code and output which is pdf format.

using Plots
using LaTeXStrings
x = -2*π:0.01:2*π
y = @. sin(x)

default(tickfontsize=12, guidefontsize=13,
p = plot(x, y)
xlabel!(L"$xxx$ variable")

Is there any method to deal with? I have no ideal with this.

Linking related thread.