tl;dr record in stereo and then split to mono
the problem here is that the scarlet solo has 2 inputs, one for your mic, one for your guitar or similar. If you only use one of these inputs, the other one will be silent, and when mixing down to mono for a mono recording, you have a simple
(left+right)/2
- or in other words
(1+0)/2 = 0.5 = -6dB