Light fixture dimming, LED turn of
Check to make sure you have the correct LED board/lamps for the driver - drivers are designed only to work with a certain set of LED boards or lamps, and if you use them outside the specifications, odd things can happen like this, or spurious lamp fail detection, or flashing behaviour.

But if the LED board is matched to the driver and this still occurs, it is very unusual behaviour and you may have to contact the manufacturer to find out why this is happening. Whilst there are strict requirements for DALI curve accuracy and monotonicity, it doesn't specify evenness of light across the fixture. It is just a poor quality fixture if the diffuser is insufficient to hide the fact that some LEDs go off at low levels.