just read this off gamefaqs, and it seems like my problem:
A few people, including myself, have maxed research for all classes, yet did not get the PhD Research achievement.
In my case, I remember there having been a few times where I would accidentally take a photo (thinking I was swapping for a weapon) and it would TAKE a photo and grade it, even for subjects already maxed for!
Working off this hunch, I went back to (gulp) Frolic and guess what? 2 photos of SPIDER SPLICERS later, I got the PhD Achievement, despite having unlocked the Spider Achievement hours before.
In any case, if you're having this problem, it's a glitch. Even if you have all the individual research maxed, achievement wise, there are still likely photo ops. I would try going back after Spider Splicers (since that was true for me), but just know it MIGHT be a different class.