When a data pack is added to a world/already loaded, if the world/server is restarted, most data pack features will not work again until /reload is issued.

How to reproduce (single player):

1. Enter a world and have a data pack added if none are added (for example, I'm using data_pack_reload_bug.zip which just runs a action bar title command.
2. Add the pack to the data pack world folder and run the /reload command (Notice no errors in game output, and the actionbar text says "Working" in black text.
3. Leave the world and come back, the text will disappear after a moment because the command is no longer running.
4. Type /reload again (shouldn't have to) to see the text pop up again.

How to reproduce (server world):

1. Load world and join server
2. Add data pack to world folder in the datapack folder and run the reload command
3. Notice black "Welcome" text above hotbar (means data pack is working)
4. Restart the server and rejoin, wait for text to go away
5. Type /reload again (shouldn't have to) to see the text pop up again.