Speed effect is missing after off-hand switching

I created a command that gives the attribut "generic.movementSpeed" to an Item. I put that item in my off-hand. In both situations the fov and the speed itself worked but then I put the item back in my main-hand.

What I expected to happen was...:
simply nothing (the fov and the speed staied normal)

What actually happened was...:
the fov and the speed disappeared as I put the item back in my main-hand. As I switched the item and switched back again it worked.
It also worked as I relogged and as I opened and closed the chat.

Steps to Reproduce:
1. put the command from the file "Neues Textdokument.txt" in a command-block and power it.
2. make the steps i have done and you can see the bug.

Greatings Leon H.-F.