I've had this situation now three times. The nectar maker is in the basement with a ladder. The sim begins making nectar and seemingly never stops. However, when I reset the sim, the bottle is ready in her inventory and she is somewhere completely different. Also I see queued up actions and even moodlets appearing while she's making nectars. One time there was yoga queued up and she even gained skill in yoga, all the while seeming to make nectar. When reset, I found her at the yoga mat. One time she got the moodlet of 'what am I doing here?' scared of thunderstorm, which was outside. When reset, she was indeed outside. Therefore I think the bug I have is not actually that the nectar making never ends, it's just that the game gets visually stuck in the nectar making animation while the sim is actually doing whatever they want somewhere else already, and the stuck-visual won't stop until you reset the sim. I have successfully done over 20 bottles of nectar, and have had the bug so far only thrice, so it's not common and I now know how to deal with it, but it can be confusing.
A thought: wondering if it's related to the skill level in nectar making, since at certain level the sim is supposed to make the nectar faster than in the beginning, and I don't remember this bug from the early skill levels. However, I haven't tested, so it's just a thought, not a confirmed thing. But maybe the animation visual gets confused when the nectar is actually ready faster than the original animation time?