That's exactly what I found out today. So I just read the manual on it and came across the electronic diagrams of the individual channels. There is a diagram for the mono channels, the stereo channels and the so-called super channel 17/18. From this diagram you can see that the gain knob on the stereo channels only works for the XLR connection. And it is true that the + 10db does work for the jack connections.
Unlike the jack connections of the mono channels, the signal path at the jack connections of the stereo channels starts after the preamp circuit of the XLR connections. The gain knob is part of the preamp circuit. So that the gain knob does not work on the jack connections of the stereo channels is ment te be by design. I tested the XLR input with a microfoon. And when you are testing you only have to connect the XLR.
It is logical that this design was chosen for the stereo channels. Normally you connect a sound source to this jack that has its own volume control. If you now use the same design as the mono channels, that means an extra gain stage. And stacking gain stages is usually not good for signal purity. Just think of an overdrive pedal in front of the amplifier input of a guitar amp. The chance that you keep the sound pure is then very small. So it makes sense that for the jack connections the signal path starts after the preamplifier, but this means that the gain knob has no function for these connections. In short, it means that your StudioLive AR16c works properly, as intended in the design.