The Focusrite Scarlett audio interface series in its 1st generation works well with Linux, and the internal matrix mixer of the devices are mapped to the ALSA mixer quite well. However, when I power up my Scarlett 18i8, the mixer settings in the interface are actually different from those reported by the ALSA mixer (alsamixer, kmix, or scarlett-mixer, etc.). Only when I actually change all matrix channel's values back and forth, the internal state of the audio interface's mixer gets updated. I phantasize that the interface does not report any mixer state, or an incorrect one, when initially communicating with the ALSA driver. A workaround in user space is to generate a mixer setting with all channels set to the lowest value, and to restore this using alsactl, and then afterwards restore the actual setting to use, this changing all matrix channel values. However, the correct behaviour would be for the driver to start the device up with proper channel values. I tested this on an Lenovo Thinkpad Laptop with kxStudio/Ubuntu 16.04 64bit. It is likely to be independent of the plattform.
Same problem on Fedora 29 64 bits. Alsa version: 1.1.8 Kernel version: 5.0.3