Can you share your code ? Because I've just wrote a small job and it perfectly works for me
InventTable inventTable;
InventTable inventTablePop;
inventTable.setTmp();
inventTable.ItemId = 'Test';
StackBase stackBase = new StackBase(Types::Record);
stackBase.push(inventTable);
inventTablePop = stackBase.pop();
info(inventTablePop.ItemId);