This is inconsistent with all other errors for reloadable resources, which fail to load that single entry, instead of invalidating the entire data pack.

Steps to reproduce

1. Add the following item modifier to a data pack

   {
     "function": "minecraft:set_components",
     "components": {
       "minecraft:custom_data": {
         "a": 1
       },
       "custom_data": {
         "b": 10
       }
     }
   }
   

2. Try to load a world with that data pack
3. Notice how you get the 'errors in currently selected data packs prevent the world from loading' message.

Alternatively you can load the world and add the item modifier later and use the /reload, command which gives you the message "reload failed; keeping old data" instead.