Fonts cannot change characters above FFFF

The bug

As of recent versions, Minecraft now supports Unicode characters above FFFF. However, it is not possible to change their textures in a custom resource pack like other characters.

How to reproduce

A resource pack is attached to this issue. Examine it and see that the font file defines textures for characters 1000 (က) and 12000 (𒀀), and maps them accordingly with a json file. However, typing character 12000 in-game results in it appearing as tofu, rather than its expected appearence. In addition, character 1000 appears to have its texture partially cut off for some reason,