I'm sorry to necro a 6 year old thread, but i think that's more on the Sims' dev team than me. This is still a thing, and your answer isnt satisfactory at all. It's "by design" and is "correctly functioning as designed", ok but like, why? What sense does that make? Why would you purposefully make a player go through this hassle, and why does nothing else in the work like this? What makes it so that this way it's better than the sim doing the full 100%? If it's "by design" then it's terrible design and should be changed, especially with people complaining.
"Should ordering this action make the sim complete the task like any other action, or should we stop it at 75%, force the sim to start the activity again (which is buried in the "web" balloon, having to click "more options" 3 times to get to said "web" balloon, if you have enough expansions, btw) and pay attention to when it changes to "complete" so they cancel that order since theres no benefit whatsoever if they finish it?" "Yes, thats a great idea, implement it immediatelly!"
Saying "nope its working as intended" without elaborating or explaining what the advantages are, or what system interaction justifies this decision, is just playing people for fools. And having to be asked "why" years after people already showed confusion about the topic, is terrible support.