I thought the primary reason that TLK didn’t work in SB was because of the “magic is different there” argument. The dark one curse is specific to the Enchanted Forest –hence why Rumple doesn’t look green in A Land Without Magic. If Belle had just kissed Rumple when they were back in the EF during then missing year, then TLK actually would’ve worked.
I dunno if I buy that. Especially considering all of the attributes of the curse are present. The only thing that is different is his appearance. It’s still a curse, even in Storybrooke. By this “different magic” logic, TLK wouldn’t break any curse there, but we’ve seen that it has. Even curses originating in EF. No, I think there is either an exact reason it’s not working, or the writers are just glancing over it to keep Rumpel’s power.