Resource pack works if there are no changes to font and even loads if there is old "pack_format", but fails to load if there are changes to font. (I'm talking about custom icons that replace text like "\uE001"...). I'm writing the code into "default.json" file that is in: ["resource pack name"/assets/minecraft/font/default.json] and takes png file in: ["resource pack name"/assets/minecraft/textures/font/"other files"]. Here is the code in the default.json file:

 
{
    "providers": [
       

{             "type": "bitmap",             "file": "minecraft:font/icons/hotbars/thirst_filled.png",             "ascent": 11,             "height": 11,             "chars": [                 "\uE001"             ]         }

    ]
}