1.13-pre7+8:
When using custom font texture files with a two color palette (like in the vanilla files), the game simply doesn't load them. The regular in-game font is displayed as the narrow unicode font instead, the SGA font only shows empty boxes.

1.13-pre9:
As of this pre-release, the game only displays empty white boxes for the regular ingame font too, see the screenshot below (both palette indices set to white), or white font inside black boxes (index 0 = black, index 1 = white, same as the vanilla font files!).

Using transparent png's with white font instead works fine in resource packs though. We need some clarification if resource packs are supposed to support the vanilla two color textures too (which would make this a bug) or if they need to use transparent background for some reason (which would make this ticket WAI). One benefit of texture files with a two color palette is an over 60% reduction in file size, which is especially useful for resource packs containing very high res fonts (I've seen people cranking the resolution up to 4096x4096).

NOTE:
The game also had an additional crash bug (null pointer exception) with the SGA font in 1.13-pre7. It  was reproducible by dropping a 2-color ascii_sga.png into a resource pack, starting the game and putting an item into the enchantment table GUI. The game would immediately crash this way.