Man,
Diverse we are going through the exact same cycles man.
I've been learning a ton of new music for an upcoming concert. For most of the music we have the lead sheets and I've been using that. Then I need to play a hym and I'm struggling with the full score. Then I'm asked to play an old song ( that I have played before) and I'm looking for the lead sheet ( when I used to remember all of my old music in my head).
I think its a function of the learning process. When you are learning new material, that is the music that is floating around in my head the most. I could play any one of those songs at the drop of a hat. The older stuff, I need to hear them run few it a few times before I can lock it in fully.
As far as sight reading, I'm still ( continually) working on that. What I have found is that the hym style of music uses a lot more open voicings than the more contemporary style, so on more contemporary music, I can immediately recognize the chord ( on the staff) than I can in the hymnal.
Also, in the hymnal, the chord is usually conctructed from both staffs where as in more contemporary music the chord is conctructed in the treble clef and the bas note is used in the bass clef.
But I'm like this, if you can sight read a hymnal, then you can pretty much sight read anything ( with the exception of maybe classical).
I/m doing the same thing you are with the hymnal, except I'm also trying to play stuff in sharp key signatures ( other than G). Like A, D, E, and B.
Jlewis