I tried using the command "/effect @a minecraft:slowness 10 5" to give me slowness effect. Although the GUI icon appears as well as in the inventory, the actual effect do nothing. Your FOV doesn't decrease and your speed is the same. Then, to check if it wasn't a problem with the command itself, I tried using the potion. Same results. Nothing happens, except for the icons. I wouldn't even write this here, because I thought you would fix it and in the meantime I could use Speed effect on levels greater than 127. Although, that didn't work either. I tried normal speed, like "/effect @a minecraft:speed 10" and nothing happened, except for the icons. I tried the potions. Nothing.

So, I really need them to work. And I know I'm not alone. If two really common effects don't work anymore, that's not good, and can be probably considered major bugs.

In the screenshots you can se the icon in the top right, but no difference in the FOV (when compared to the first image)