Adding item to dropbox deletes previous entries

There are two obviously related cases:

• when I am creating node, I have to first select an item in drop down, add it to dropbox and then repeat the step - because for the fist time item was not added to selection (dropbox)
• second case is when I edit a node, adding a new item to dropbox deletes all of previous ones (but again does not add that item to dropbox)

So I am guessing the first click on Add button resets/deletes entries from drop box, and only the second time it actually adds it.

I am using this on Term Reference field on node.