Changing characters with a resource pack in a unicode_page_XX.png file located under assets/minecraft/textures/font at least sometimes does not change the characters with, so making a character smaller than the original one, will result in a gap, while making a bigger one will cause it to get cut of. For example making the unicode_page_aa.png file completly white will cause all the boxes to be different withs and looking at the original characters, the sizes match. If there is no character in the original unicode wile, that the character will also be displayed as a box, even if it has been added with a resource pack. For example everything in after unicode AA37 (AA38, AA39...) will just be a box, even if it should be something different.