It has been now confirmed by users around the world that the Mitsubishi Outlander PHEV all models' battery is degrading at an unrealistic pace. It is suspected that the Battery Management Unit (BMU) has a faulty firmware which calculates the battery state of health (SOH) incorrectly. 
There is a YouTube channel run by Andreas Kirch who is trying to analyse the problem and find the solution:

https://youtu.be/HSD3HlKVs1k

and by Garry Reed:

https://www.youtube.com/watch?v=41gwlZ2JKVg

The data obtained by users come from OBD2 adapters and are relayed in a human-readable form by the PHEV Watchdog application written by Daniel Santos:

https://phevwatchdog.net/

Unfortunately, viewers are reporting that the Mitsubishi dealers around the world refuse to acknowledge the problem and claim that the degradation is natural. That statement is false. 

We demand from Mitsubishi Motors Japan to find the solution for the faulty BMU, update the firmware and recall all the models for the BMU upgrade as well. Failing to do so may lead to a local and/or global class actions against the Mitsubishi Motors representatives.