They changed the dynamics of when plants grow and are harvestable in relation to the time of day besides just what season in which they will grow. You need to be in the area before 5 a.m. when plants grow produce/flowers. At least that's the case with certain venues such as the lab for the Scientist career. That's why Scientists have to grow their own plants now to create serums because the job starts at 10 a.m.
Now, I don't know if that's true for everywhere though, for instance in parks and neighborhoods where you don't currently live. That's the other thing, once you do plant something on your lot, you need to be home at 5 a.m. in order for the plants to grow. If your Sim is gone somewhere else, the plants won't grow anything to harvest until you are there around 5 a.m. (could be 5:30-6).
Plus, grafting has been borked ever since the gardening patch before Seasons. The percentage is much lower (like 1%) that you'll ever get the produce you grafted to get. That's one BIG reason why I don't garden as much as before that patch. It's a pity since that was one of the things I loved doing most in my game.
Anyway, if you're determined to get Dragonfruit, all is not lost. Just go fishing! The pier in Magnolia Promenade is one spot. There's a very high percentage your Sim will fish up at least one, probably more depending on how long you let them fish there. I got 3 within just a couple hours. Also, Magnolia Blossom park's fishing spot way in the back where there are three signs is another place you can get Dragonfruit. Just make sure you fish at the sign in the middle, that one is for 'rare' fish and the only one I personally have been able to get Dragonfruit from.
I hope this helps! Happy Simming! <3