With Android 6 (Marshmallow), Google has introduced Doze mode to the base Android, in an attempt to unify battery saving across the various Android phones.
Unfortunately, vendors (e.g. Xiaomi, Huawei, OnePlus or even Samsung..) did not seem to catch that ball and they all have their own battery savers, usually very poorly written, saving battery only superficially with side effects.
Naturally users blame developers for their apps failing to deliver. But the truth is developers do the maximum they can. Always investigating new device specific hacks to keep their (your!) apps working. But in many cases they simply fall short as vendors have full control over processes on your phone.
This is a legitimate problem on my OnePlus 6T. I enjoy using this phone, but the aggressive non-standard application cycle management definitely leads to issues with not receiving notifications or login procedures being restarted as you leave an application. It doesn’t happen often enough to truly bother me, but I can definitely see how people who make more extensive use of their phone than I do run into this issue every day.