From Wikidata chat:
When entering in a property or its corresponding item, if you were entering in the underlying identifier, that is typing "p31" or "q3331189" "q1860" the code lookup would be case-insensitive. Overnight—presumably during code update—such actions have become case-sensitive, meaning that it needs to be "P31", etc.
This needs to be fixed, there is no reason for case-sensitive match.