Seems kind of frustrating to think you’re locked in then lose 53 points a couple minutes later.
Yes, it’s the delay between your device syncing with the server after you shield. Other fights may have checked into the server before your shield did. This can be remedied slightly by forcing the device to sync with the server by checking your placement immediately after shielding. I even check twice. Going to the rewards, and checking individual placement forces your device to connect to the server to get an update. You may still get a hit during that tiny window, but much less of a chance than waiting for the device to decide to sync up on its own.