When a property has 'item' datatype, this can be entered either by directly typing the item ID, eg Q123, or by typing the item label or alias, eg September.
Pasting in the Q-number from somewhere else is a pretty common practice when entering values, especially if the item has a very common name ("John Smith") or doesn't have a label in the currently used language.
Unfortunately, most uses of item IDs on Wikidata show the ID in brackets - September (Q123). This means that copying-and-pasting often picks up a surplus pair of brackets that have to be removed. Would it be possible for the system here to identify (id) as a valid ID and accept it? A number of external tools (eg Magnus's mix-and-match) can handle bracketed IDs and it would be good if Wikidata proper could as well.