Although the sorting button can sort the items in the target inventory immediately, the keyboard shortcut cannot reach that. (I should say, the keyboard shortcut actually sorts the inventory, but the inventory does not immediately update.)

1. Before sort and before reopen the chest
   
2. Sorted (Using keyboard shortcut) but before reopen the chest
   
3. Reopen chest
   

On further inspection, this is a dupe of #1782

Can't reproduce, it always sorts instantly for me.

I just tested it more thoroughly and sorry for my carelessness yesterday.
If I set the "middle button" to sort the target inventory, and at the same time, I set the "middle button" to pick block.
These two actions, I personally think, do not conflict with each other.

However, the sorting function will just like what I described above if I set "middle key" both for sorting and picking block.

Also, if I set the "middle button" to do other unrelated actions like throwing an item which does not conflict with the sorting function because we cannot throw an item in a chest GUI. However, the sorting function will still behave like what I described.

To be more clear, I think this may not a bug and this is more like an enhancement.