The bug

Bedrock provides character texture sheets for the following unicode ranges:

• U+0000 - U+07FF
• U+9000 - U+D7FF
• U+E000 - U+E1FF
• U+F900 - U+FFFF

This means the following ranges are not included:

• U+0800 - U+08FF
• U+D800 - U+DFFF*
• U+E200 - U+F8FF

One would expect that if you wanted to texture those characters, you could simply include the missing files in your resource pack, e.g. font/glyph_08.png. However, this does not work, and including that file results in the characters still being missing.

As a result, it's impossible to provide textures for some characters in bedrock minecraft.

(* note that this range is reserved for surrogate pairs so it makes sense that this range is excluded, although Java is fully capable of retexturing those pairs so there's no reason Bedrock shouldn't be able to)

How to reproduce

Download the attached world

/tellraw @a {"rawtext":[{"text":"\u0100"}]}
/tellraw @a {"rawtext":[{"text":"\u0800"}]}

Expected results
The characters are identical since the sheets glyph_01 and glyph_08 in the resource pack are identical
Actual results
Second character shows as a box question mark

Notes

If bedrock had data-driven fonts like java minecraft, this would not be an issue since you could point any character to any resource pack texture file. But we all know that will never happen.