A loss disguised as a win is a spin where the game pays something, but less than you staked, so you are down overall. A £1 spin that returns £0.30 is a £0.70 loss, whatever the screen says.
On games with many paylines or ways to win, small line wins land often, so a high hit frequency can hide a steadily falling balance. Historically, these were often greeted with the same lights and sounds as a genuine win.
The Gambling Commission's game design rules that took effect on 31 October 2021 ban GB-licensed online slots from using sounds or imagery that celebrate a "win" equal to or less than the stake. The same rules require the game to show your net position, so you can see whether you are up or down rather than relying on how the game sounds.
The simple check: compare the win with your total bet. If it is not bigger, it was not a win.