I change the config for both wither and ender dragon, wither did drop loot 100% but dragon are not dropping loot
"minecraft:entities/wither": 1.0,
"minecraft:entities/ender_dragon": 1.0,

You probably have loot tables that are overiding the loot inject. You would need to find whatever datapack / mod is adding hte loot tables and remove them. This is the only way to resolve the issue unfortunately.