I noticed the same burning plastic smell when the transmission hesitated and lurched after a rolling stop. That odor hit me immediately after the transmission hesitated/lurched. This happened with about 50 miles on the odometer. I bought my RAV4 with 7 miles on it and there wasn't any burning odors when it was at the dealership. There is definitely something defective going on. It is not normal for any car to have this type of odor when it's new.
I drove 25 miles around town to find out when my RAV4 emits this burning odor and it happens when the transmission is downshifting after coming to a near stop. If I stepped on the gas at 1-2 mph the transmission would lurch and the burning plastic smell would get stronger. Now I avoid rolling stops and the transmission shifts normally off the line. I'm an electrician and this burning smell reminds me of insulation that is overheating and melting at a very high temperature. Something around the transmission is getting way too hot and causing this material to burn. I'm not a mechanic but I know there has to be something wrong when a burning plastic smell goes off in a split second. I'm going to look under the hood and underneath the engine bay to find out if there is anything that looks charred.
Take it back to the dealership to have them look into it. If they can't reproduce it, smell it, or deny there's a problem, call Toyota Care and have them document the issue. Also request that a Toyota Specialist come out and take a look at the vehicle. If you keep having the problem and the dealership/Toyota is not able to remedy it, start thinking about having Toyota replace the vehicle under the lemon law. The representative at Toyota Care should ask you what you're expecting from Toyota, but if they don't, tell them that you are asking that Toyota fix the problem or replace the vehicle. There's a pamphlet that is bundled with the owner's manual and other booklets that describes steps you need to take in these situations.
As for me, I've just started that process and am waiting for an appointment to have a specialist come out to the dealer to examine it and figure out a solution. I have a handful of issues with this vehicle that I am going to document in another thread, but I will say here that I am having issues with the transmission. It "thumps" when coming to a stop (at around 3-4 mph) when the vehicle engine/power train has not yet come up to full operating temperature (before 10-15 mins of driving). I have also noticed hard shifts while driving at higher speeds, and also jerky, whiplash types of effects with "rolling stops", like you're describing, particularly when the vehicle is cold. I haven't noticed any pronounced burning smells when changing speed, although I have recently noticed that the odd "cedar wood" type smell inside the car seems to make itself known after harder acceleration, or after slowing down. Maybe it's the same as what you're experiencing, but in my case it smells different after making its way into the cabin from the engine compartment.
My dealership has been of no real help to me, and in fact the service manager tried telling me that the other two new '19 Ravs that we test drove were doing the same type of hard shift into first gear that my car was doing! They were NOT anywhere close to the same effect. They had a very "slight" sensation sometimes that you could feel when coming to a stop, but you could not hear it. Mine has a very pronounced thump feel and sound. Ridiculous!
BTW, mine is a J-spec (Japanese built). What is yours?