It means you probably have a few missing rivets in that area and the plastic top plate is separating slightly from the back metal plate, letting that flipper come off its "mounts".
You're going to want to open up the keyboard and take a look at the bottom of the keyboard assembly.
The rivets are basically little plastic stems that are pushed down ("mushroomed") to hold the metal plate and the plastic barrel plate together (with the membrane sandwiched between them). When they fall off the plate can get loose.
People do "bolt mods" where essentially they drill out all the rivets and replace with tiny bolts/screws. If your problem persists you can try getting away with just drilling out the ones that have fallen off near the spacebar without disassembling the whole thing. There's a small risk of debris getting in when you do that so you'd have to do a full mod in that case.
Or you can return it to the seller as long as it wasn't an as-is purchase. Which this old keyboards usually are.
