UR44 inputs 3/4 are demonstrably quieter than the others

So not particularly sure if it’s always been this way or if I just noticed it now, but when I plug something into inputs 3 and 4 (for instance, a volca or pocket operator), it’s much quieter than the others, and more gain is required to get it up to the same level. Swapping over the synth to another pair of inputs (either 1/2 or 5/6) makes it immediately louder. Not entirely sure why this would be the case, really. There is some button in the DSPMixFX app which seems to make it a touch louder, but I’d like to find out why this app would be affecting things, if indeed that is the reason why.