It's not a problem of Android version. Apps are compatible with Android 7 and I've tested it with other roms, it works properly. There are system sounds in silent mode except screen-on and screen-off sounds. I mean every time I touch the screen, every time I use a keyboard and every time I make some actions, which have a system sound I hear it, though sounds turned off. And it's not only my problem. Every Mi4s owner on two last builds by xiaomi.eu have this.
And I repeat, there are no problems on official roms and roms by multirom. I'm respect your work very much, but I don't understand, why you don't want to hear a feedback.