I think you actually have to have one person doing all 4 plants. I had a similar experience where I had one villager do 2 plants and another do the other 2 plants, and the puzzle remained unsolved. I didn't keep that village around much longer, but for as long as it was around I could never get either villager to gain knowledge about the two plants they'd missed. -_-

I think you might be correct. Probably by the time all the herbs were mastered the puzzle didn't solve itself because more than one person did this and therefore later when I tried one person checking out all those plants it didn't matter because they'd already been mastered.

I've had different villagers study different plants and the puzzle always solves.

When I start a new game, I'll put four villagers on each plant at the same time. That way when the last one is finished they aren't in the middle of a task when they stop and celebrate.