When you drag the box from where the items are inserted to the block, they disappear. The problem arises when we need to add more items that take up more space than the height of the screen.
You can show us (screenshots/video etc) what you mean for us to be certain, but i believe MIT are already aware of this issue, which is related to the recent blockly update.
I like the free ScreenToGif program to capture short screen recordings for these.
They are small enough to upload, and explicit enough to prove to MIT what's wrong.
Thanks for the report.
It has been passed to MIT.