Based on a report by DontKillMyApp.com website, OnePlus and Nokia are being too aggressive on background applications, as their phones seem to be killing background apps too easily in order to save battery life.
The aforementioned website actually lists Nokia as the main offender. In fact, Nokia was the main reason why the DontKillMyApps website was created, at least according to Jiri Richter from the Urbandroid team. According to the website, HMD Global actually uses a "battery protection" application in Android O and P in order to kill background processes 20 minutes after you turn off the display.
The main reason, not the only one though, of having plenty of RAM included in a device is so that a phone can hold apps in memory, instead of having to reload them almost every time. Users expect to be able to open up an app that they closed a while back and being able to pick off where they left off, though unfortunately, that's not the case in a ton of situations, as either OEMs or apps themselves prevent such scenarios.
According to the source, Nokia is the main offender, but OnePlus seems to be a close second, and is followed by Xiaomi, Huawei, Meizu, Sony, Samsung, HTC, stock Android, and "other vendors". Now, that's not all, this website also includes explanations of how each OEM limits background processes, and how you can try to amend that, both users and developers (if it's an option).
This website was actually launched quite recently, and it's already getting plenty of support, even outside the Urbandroid team. The developer behind Tasker and Join has officially voiced his support for the website, amongst others, as it seems like developers are kind of frustrated with OEMs when it comes to limiting background processes.
Now, there's usually a good reason why OEMs set such limitations, as they want to save battery. The vast majority of users will not even notice such aggressive termination of background processes, but those that do are left frustrated by OEMs choices. At the very least, OEMs should give users some sort of an option, just in case they want their phones to keep those apps in the background for longer, even if that means less battery life, though optimization is the key word here, as this should not be a problem at all in this day and age.
Google had released several features to preserve battery life in Android OS to date, and the latest one is called "Adaptive Battery", and it controls background apps, preventing those that are battery-hungry to do much damage. OEMs should utilize Adaptive Battery, and maybe even build on it in order to prevent aggressive apps terminations in the background, so that users can properly multitask with their devices, without having to reload apps, no matter how fast that happens, as it's always more convenient to pick up where you left off than having to navigate to a particular part of the app once it's loaded again. Let's hope that this website will affect OEMs at least a little bit, to get them to thinking.