Different icons load randomly

I’m trying to customize my Polybar using icon fonts and have encountered a problem.

When I copy/paste an icon from a cheatsheet into the config (I’m using vim) and then load polybar with it, sometimes I get the one I copied, sometimes it shows a completely other one. Vim always shows the same one as the polybar.

I’ve used (tried to use) Font Awesome and Material Icons, both installed using pacman.

I can post the config if you’re interested (though I don’t think it matters, since in the terminal they behave the same)

PS: I know it’s quite a common problem, but I haven’t found a solution that worked for me.