@Anton Lakes It seems to have stopped happening for me, so I thought that is was resolved, but one thing I did do before the update (I didn't play after I did it before the update) was I turned off the paper doll option, so if you have that on try turning it off and see what happens
It was fine for a while, so I updated the description to say it was resolved. It then started happening again and I forgot to update the Description until I saw your comment. Sorry about that. But It has carried on to the Better Together Update too. I think it might have something to do with performance problems (With the game, not my computer because it handled the game normally before, and with adjusted settings to the lowest it still happens) because before this bug came delayed keys seems to be this games way of having lag when too much is happening. Hope this was helpful 🙂
That's why I posted this, because a limited amount of items can go into the offhand in bedrock, so fireworks being able to makes me think that it's supposed to work, yet it doesn't. So hopefully this gets fixed in a way that makes it work, rather than it not being able to go in the offhand, because it is a really nice feature that is only currently working on Java.