a month ago I created a modpack with my friend but, when we started playing it, we discover items that we picked-up (like cobblestone or oak wood) didn't apper in our inventories.
My questions are: Is this bug caused by inventory tweaks? if it is, how we can fix it?
Thanks for answers and sorry if my english is not so good (I'm not english)

It's probably possible that the inventory isn't being sent over properly in some cases, but I haven't seen anything of the sort happen myself.

Do the items reappear after reconnecting or when clicking around random spots in the inventory?

they reappear when I click the inventory sorting button (by default the middle button of the mouse) or when I right-clicking with something that has a durability in the hand.

i solved: the bug was caused by optifine...

Optifine does some strange things, but I can't say I've heard of it doing that before.