Given a ttf file used as a font, it is clear that the system is rendering the text against a white background and then painting the non-white pixels into the scene. Which leads to really ugly, frankly illegible text on signs, where the background isn't white. See attached snapshots

Here is my default.json file, using a ttf file for Arial Unicode:

{{{}}
{{ "providers": [}}
{{ {}}
{{ "type": "ttf",}}
{{ "file": "minecraft:arial_unicode.ttf",}}
{{ "shift": [0, 0],}}
{{ "size": 13.0,}}
{{ "oversample": 10.0}}
{{ }}}
{{ ]}}
}