Android in stock version, albeit in a different way than what the customizations of other manufacturers propose, as we all know includes an energy saving mode that is activated in case of low battery life. The threshold can be changed by the user but at the same time can also be set by Google.
The manufacturer, in fact, by varying some parameters within its own servers, can modify the residual charge threshold necessary to activate the energy saving.
Just this (strange) flexibility chosen by Google has been in recent days due to a rather unusual problem for Pixel, Essential Phone, Nokia 7+ (Android P beta) and OnePlus 6 (Android P beta) users: the energy saving mode was activated when the residual charge was less than or equal to 99%.
Google has rectified the error in a few hours, but the fact is that many users have seen their smartphones in Battery Saver mode, then with degraded performance and a limited number of processes in the background, without any apparent reason.
The producer then apologized and explained on Reddit that it was an internal test that was erroneously distributed publicly.