Burning rubber smell from back of car, while just driving on ECO.

[Phev2016]

"the smell came during engine running and charging" which is when the engine and exhaust will be hottest and likely cause of smell. If your transmission (or whatever word is now acceptable on the forum) was creating a smell you'd have been in real trouble and an oil change would be unlikely to fix it.

 

[Haenzel]

It's usually just stuff on the exhaust pipe burning off because the PHEV exhaust pipe gets hotter than a normal ICE would generally, as you get high engine output at very low speed where there's no cooling airflow, whereas you can't do that in an ICE without driving against the brakes.

This is not a new hypothesis, but it's been oft dismissed by people that don't understand it or haven't considered my point above.

How do you explain the smell is still there a day after having driven 1200 km and having crossed multiple steep low speed mountain passes where the ICE has worked like never before?

 

[Phev2016]

If it isn't from the exhaust surface (and there is no exhaust leak near plastic or an incorrect mounting etc) nor the brakes and it's too overpowering and persistent to be a drivebelt, alternator, wheel bearing, driveshafts or similar, it seems like you are left with main battery, power control modules, engine, electric motors, generator or transmission. There is an electrical heater on most models which should be immersed in water so that's unlikely!.
If this were my car the approach my logic suggests is to begin from cold and drive at a steady speed in EV mode only until the battery is low, but without starting the engine and then check there is no smell. Next, recharge from 230V and repeat a cycle of good acceleration and slowing down with Level 5 regen as best you can, but ensure the engine does not start. If no smell then that rules out quite a few things and I suspect you already think this is the case.
Next, having reduced the battery to about 20%, switch on from cold and use the vehicle Charge setting when stationary and after about 20 minutes of engine running it will reach 80% charge and stop. If it smells then unlikely to be the emergency dry clutch as it would not be transmitting high torque and unlikely to be the rest of the transmission either but I'm less sure. If no smell then probably not the generator but it (and possibly it's power controls) does get hot and emit a 'hot motor' smell rather than a burning smell in this mode.
So then it's either engine or transmission. If there's no smoke from the exhaust, the engine oil and the coolant are clean (with no leaks of either) then you are left with the transmission.
An earlier post commented that it could be incorrect oil. It might also have been overfilled which can cause overheating in some transmissions if designed to optimally splash oil rather than have total immersion. Changing the oil is relatively expensive and it can be measured for volume, examined for debris, and ideally sent for analysis (relatively cheap) which will determine if it's overheated, exhausted or contaminated.
If all that does not find the cause then I guess you should see a good PHEV technician, otherwise although it seems unlikely it may be the emergency dry clutch which would be expensive to repair.
I hope this helps. I wish you good luck.