Feeding a cowplant only satisfies hunger if the "cake" is visible.
Product: The Sims 4
Platform:PC
Which language are you playing the game in? English
How often does the bug occur? Whenever you feed a cowplant before it's hungry.
What is your current game version number? 1.65.77.1020
What expansions, game packs, and stuff packs do you have installed? Sims 4 Seasons, Cats and Dogs, Get Together, Jungle Adventure, Parenthood, Spa Day, Outdoor Retreat, Tiny Living Stuff, Perfect Patio Stuff, Realm of Magic, Island Living, Romantic Garden Stuff, Nifty Knitting
Steps: How can we find the bug ourselves? Have a Sim get a good night's sleep, and feed the cowplant in the morning before the "cake" is visible (where the options are "Feed," "Pet," or "Play With," not when the options are "Feed" or "Eat Cake.).
What happens when the bug occurs? The cowplant becomes hungry and sticks it's Cake out almost immediately after!
What do you expect to see? The cowplant to have its hunger topped off and not to get dangerously hungry.
Have you installed any customization with the game, e.g. Custom Content or Mods? No.
Did this issue appear after a specific patch or change you made to your system? I think this happened after the latest patch. (The one with the much-desired pets eat also not on command!)
I've had two cowplants die since the latest patch and was feeling really guilty and neglectful, but I've noticed in my renewed desire to not kill any more that feeding doesn't seem to work unless they're in the Cake phase of hunger.
Okay, with further testing I'm not 100% sure my initial observations hold. I still think they're getting too hungry too fast! and faster than they used to! and did have a cowplant stick out its tongue IMMEDIATELY after I fed it without the tongue visible, but... it instead seems to be completely random where instead before I thought there was more of a pattern. So I'm going to mark this as the solution (such as it is).
(Basically: Feeding not topping off hunger is probably a bug. Changing the rate of hunger decay to possibly be an RNG rate is probably an intentional development choice.)