Interesting information. I'm not doubting you here but want to clarify one thing, to understand. After you contacted Oneplus and were added to whitelist, do you still see that your app is showing as "optimized" in the settings but app behavior changed and it is no longer killed in background? And that same is applicable to other major software?
Though they were moving away from that model to a model where apps alive for more than a certain time interval in background were killed. So apps which you use the most heavily are more likely to be penalized.