As someone who has been on all kinds of drugs since I was 14 (epileptic) I can tell you its very common with all kinds of medications to have different side effects fade in and out over time. For example the med I am on now use to cause me to get horrible dizzy spells when I was in high school that sometimes prevented me from moving at all for a few hours. For a long time I had no sides with it, and just in the last year or so I have been experiencing memory loss sides that are causing me to change meds now.
Basically meds respond to your own personal chemistry, and its also true meds may not be as effective as they use to for various reasons with your body. But you should not assume a loss of side effects is a sign of the finasteride is becoming less effective. This is a moment of celebration for you! Disappearing side effects, yay!!