Assuming it's a pressed headset and not some kind of integrated version, a cup simply dropping out is less than ideal. But it isn't all that rare on used bikes because of things that can happen.

The most likely (and best from your standpoint) possibility is that someone used a JIS 30.0mm cup in an ISO 30.2 headtube. That's an easy fix, get the right headset.

The other possibility is that the tube is oversized. If it's now JIS, ream it properly to ISO (both top and bottom). Otherwise, the tube can be "shrunk" by painting the inside. Nail polish works fine for this. There's also a bearing-grade Anaerobic (Locite) that swells as it sets, made specifically for this kind of thing, but it's pricey and overkill for one time use. Over the years, I've had very good results with paint or nailpolish.