Pomegranites not growing from cherry/apple grafted trees?

No it's not a bug they are rare to sprout even on a spliced plant. They changed the chance of getting them when they revamped gardening.
I'm in the same boat.. still waiting on an orchid on one of my grafted plants and it's been many, many weeks. I have better luck getting pomegranates fishing. Not positive but think you can also get them at the spice festival if they are in season and you have City Living of course.
You can also try buying rare seed packs (or getting them from Patchy) if your sims gardening skill is high enough to unlock that.
I'm refusing to "cheat" for my sim in my current play through but you can always download seeds/plants from the gallery as another option.

Oh yes, I forgot you can get stuff from the festivals. Well, all those annoying pop-ups that I keep ignoring might have been what I was looking for all along. Won't be so fast to dismiss them next time I see them. Luckily ghosts don't age so they will wait till my living sim gets the ambrosia made (I do cheat with my living sims and de-age the ones I like and keep them young...the rest of the neighborhood is on it's own though).

They only can be harvested in the winter.

I've had great luck fishing up many Pomegranates from the pond in Sylvan Glade (accessed by the mysterious tree in Willow Creek). Also, I'm not 100% positive about this, but I think they might also be available from the 'Purchase Gifts' option on your Sim's cell phone. They might also be available to purchase at a Ranger Station in Granite Falls if you have the Outdoor Retreat GP.

As @Hermitgirl shared, the chance of getting a rare plant from grafting was reduced to only 1% in the gardening update patch before Seasons dropped. That means you only have a chance of getting one after harvesting the grafted plants around 600 times. o.O IMO grafting for anything rare would be a last resort for me. For instance, it took me almost a year over many saves with many Sims to get ONE Orchid from a graft.

Good luck and Happy Simming! <3

I only did this once. Try grafting your Cherry/Apple tree to a Lemon Tree which grows all year long, then harvest it in the winter.

If you have City Living you can buy pomegranate from the produce vendor they may also be available in the Buy Gift menu if you have Seasons. Also as someone mentioned you can fish them up.

If you have the Romantic Garden Stuff you may be able to obtain the ambrosia recipe items from the Wishing Well. I believe the well will also possibly just make your sim not a ghost if you give it enough money and/or it’s in a good mood.

My sim planted and grafted 3 trees as a young adult, playing on long life span they aged up and died without ever getting a pomegranate from the trees (including a lemon base graft). If the chance of it working is so ridiculously small as to be rendered useless, why does it still exist at all? It was disappointing that after all my sims hard work gardening - that it was so easy to buy and find pomegranates elsewhere with no skills at all.

Cool. I have all those packs so I'll have to check them out. No way I'd have known about all those ways. Now she needs to find that last space rock.