Incorrect Ikonli icons displayed

I am having problems using the Ikonli icon packs. I am creating FontIcon objects and then using those to set the graphics on buttons. Icons taken from the FontAwesome5 pack work great. Icons taken from the Material2 pack seem to shift somehow - for example, I try to get mdal-6_ft_apart but what actually gets displayed is mdal-loyalty. And no matter which Icon I load from MaterialDesign2, all I see is an empty box.

I'm running this through Eclipse, and everything looks fine when stepping through with the debugger. This is the value of the button graphic:

ObjectProperty [bean: mdal-6_ft_apart:15:0xffffffff, name: iconCode, value: MDAL_6_FT_APART]

and yet, on the screen, it's something completely different.

This is a Maven project using OpenJDK 16 and JavaFX 16. I've tried loading just a single icon pack instead of all 3, but no difference. Also, there are never any errors retrieving the icons. The ServiceProviders are all found, and the icons are available.

I made a small Hello World program and everything runs fine, so it's definitely an issue with my program and not the library. But I have no idea what the problem is, and I've been staring at this for almost 2 hours. Has anyone come across this before? I'd appreciate any help, because I'm out of ideas.