Iosevka | Versatile typeface for code , from code | User Interface library

I'm trying to write a small XSL-FO file with absolutely positioned elements. One of them is a label in the corner. I've managed to get it pretty close to but not quite what I want. Here it is:

I want to center the text vertically and horizontally. The horizontal position is OK, but the vertical is maybe centered according to the font metrics, but visually it's slightly to the top, so I'd like to move it a couple points down. But how do I do that? Here's the XSL-FO file:

I've been using airline in Vim for quite some time now, with a variety of different fonts. However, I recently switched to a new machine, and I can't seem to get my powerline glyphs to work with any font except for an explicitly patched Monaco (from the monaco-powerline-font-git AUR package). I have tried a couple of other fonts, such as Hack and Iosevka, which ostensibly have Powerline glyphs built in and, as far as I can tell, work out-of-the-box for others.

For comparison, here's what it looks like with the patched Monaco:

As you can see, all glyphs are displaying normally.

Here's what it looks like with Hack and Iosevka:

The Powerline glyphs seem to have been replaced by a bunch of ugly placeholders.

I don't mind Monaco too much, but I would prefer having some flexibility in choosing a modern font with built-in Powerline support.

Does anyone have an idea what I need to do to fix this?

Here are some system details, although I don't know which of these are actually relevant:

OS: Arch Linux
Editor: Neovim
Terminal Emulator: Termite (but the issue is identical in gnome-terminal as well as GVim)