This is opus 60 number 9, and it's one that I play frequently. Numbers 8 and 9 in this opus are particularly fun for me because I play them fully respecting the stem-down = p, stem-up = i/m technique, and I really enjoy this.
I have my own interpretation of perhaps a reason that particular measure (7) appears to be notated differently, and it has to do with Sor's technique of the thumb. Granted - I'm a beginner musician, studying outside of a formal academic environment so my "interpretation" is really the impression I get by performing a sort of amateur intellectual exercise of reading between the musical lines. I think I've probably branded myself as a little bit of a nut job because of how I've been approaching Sor's studies, so take that into account.
My thinking is that one of Sor's goals with this exercise and the one preceding it was to have the student experience the use of the thumb, it's different tonal quality, and it's particular quirks of control. For me, the challenge has been to have a dynamic difference between a strong bass note that sustains appropriately, while playing a softer middle voice also with thumb. In several places the middle voice is shared by a thumb and then an index, and experiencing and controlling those tonal differences has been valuable to me.
To me, then, the use of eighth notes vs. quarter notes isn't just about acknowledging a length of sustain - because we already let these notes ring. It's a dynamic difference. Getting into the mindset that measure 7 is to played with notes that would ring longer is a sneaky way of inducing a crescendo.
Anyhow, that's how I play it, and I like the way it sounds! Your mileage may vary.
"If I have seen further it is by standing on the shoulders of giants." -Sir Isaac Newton
Armin Hanika 56PF