WordPress Icon Problem – Some icons is not showing

Some of the icons have been missing for a while. Instead of the icon, it looks something like the image:
Image Link

I thought it was from the cache plugin. But the problem is not here. I disabled all the plugins but the problem still persists. I searched but couldn’t find a definitive answer and couldn’t solve the problem. Why can this be caused?

For example, I’m trying to add an icon with the following simple CSS code. However, in such a case, the F4FE text appears as in the image. So the icon does not appear.

.woocommerce-MyAccount-navigation ul li.woocommerce-MyAccount-navigation-link--dashboard a:before {
    content: "\f4fe";
}

The strange thing is another example. If I define a different icon code like this, this is what it looks like.

.woocommerce-MyAccount-navigation ul li.woocommerce-MyAccount-navigation-link--dashboard a:before {
    content: "\f0e4";
}

Also, I can’t see some icons defined by plugins. F4FE text appears as in the first image. I would be glad if you help. I couldn’t find a solution for days.