Sorry Borg, but I'm still getting no credit for plants that I have got and it's not just Magic Plants either for me.
I did a little experiment a while back where i planted a seed and used miracle grow on it and kept checking the stats page. it seems like the game only checks-for-new-plants/ updates-the-stats-screen every few minutes rather than instantaneously. also, it does not seem to give credit for new seeds, they have to be planted (and perhaps even mature? i can't recall).
It is easy to do an experiment like that. Start a new game. Visit the stats page and it should say 0 species discovered. Plant some seeds, keep checking the stats page. If you have the same experience I did, it seems to take forever to get credit for the plants.