I might have been the first person in this thread to discover this.
I was playing a "homeless sim" who was a single mother. She lives on a 10x10 "vacant lot," in which the only two pieces of furniture she has is a park bench and a street light. She has lived on this lot since she was a teenager (I moved her in there with an adult and then killed the adult off and reduced household funds to 0), and every simolean she has she has mooched off other sims. Once she aged up to adult, I had her befriend/romance the first male sim she met and got her pregnant, just to make things interesting.
There is literally nothing for her to do on her lot for to see to any needs other than sleep (and she naps on the park bench for that), so she has to go elsewhere to fulfill them. When her child was an infant, it was problematic for her to take an infant places, so she left her daughter at home when she went out to mooch food, watch tv at the gym or bathe in the various sinks around town. Thus, she hired a babysitter.
Any time she would leave the lot, my attention would be on her. However, the baby's needs would drop so low that her picture on the side of the screen would turn bright red. When I clicked on her picture to see what the issue was, her social, hunger and hygiene needs were bottomed out. Likewise, I'd get a warning occasionally that if I didn't see to her hunger soon, social services would come and take her. So I think "what in the world is that babysitter doing?"
When I pan over to the lot, the babysitter is standing beside the mailbox, doing absolutely nothing. I'd send my sim home to fire her, but shortly after the camera settled over the lot, she'd jump to pick up the baby and feed her. After the same thing happened a few days in a row, I started noticing the pattern. So I started experimenting with her like you did with the maid. Literally any time the camera was not on her, she'd stand around and do nothing. The moment it was, she started working.
I figured this was a problem exclusive to Bridgeport, since I had never encountered it before playing there. However, after I discovered this, it has happened in every town with every babysitter, without fail. However, I've never had the opportunity to test this with maids, since I usually have a "Neat" sim in the house to begin with.
Thus I have to wonder if this is a bug, or if the game is simply unable to handle autonomous behavior when the camera is off watching another sim.