Update: How the heck is no one else having this problem!? Seriously, I only know one other person who has this problem.

I recently tried to use some addons that had custom items and blocks (not overriding any other vanilla IDs), and all of these were for 1.12 beta for that reason. I installed the addons on different worlds, and the addons say they installed just fine, but when I load up any world with those addons installed, only the vanilla blocks and items show. No custom IDs to be found anywhere, and according to reddit, I'm not the only one having this problem. This happens with every 1.12 beta addon I use that has custom block and/or item IDs, but what really gets on my nerves is that there are screenshots that show these addons working just fine! What is going on?