Hi MrsFlynn from 2012, this is MrsFlynn from the future *Smile* (Feb.2020)
We ran into the issue again, but didn't try a whole lot of troubleshooting. However did find that reset everything via
Nraas MasterController doesn't work, neither does MC's Reset Lot. I saved the game, and then quit to main menu to try a test save.
I tested the house/gardening lot in the new test-save and the Sims were able to tend garden just fine. So I quit without saving, and went back into my main save and was able to get my Sims to tend garden again. -- My best guess is something hiccuped while playing.
If you run into this issue again, try this: Save, Quit the game and reload the game.
It works wonders. *Blush* Just wanted to leave the tip, just in case it happens again.. and we're searching for answers. lol