Changing sound balance of external speakers to left or right causes the sound level to mute on both sides

My Mac mini 2023 (M2 Pro, running macOS Sonoma 14.4.1) is connected to a Creative Pebble V3 speakers over USB-C. Under normal circumstances, the sound comes out normally: in stereo, from both left and right speakers at the same level.

However, if I go to the macOS System Settings / Sound / Output & Input / Balance and move the sliders from the centre to either left or right channels the sound level (of both speakers!) decreases gradually as it approaches either side and eventually mutes completely (when I have the slider at 100% left or 100% right).

Are there any suggestions to investigate and fix this issue?

Thank you!