Try Flashing the stock boot image, To get the stock boot.img of PE, download payload dumper (Attached) and do the following steps:
It's not about alert slider, that's aosp behaviour. In silent mode media volume isn't supposed to be off. OnePlus changed it and it's not my job to match stock behaviour everywhere. If that was the goal then you can use OOS.
Actually, after the OTA got downloaded and installed, it showed a pop-up with reboot button.
OnePlus released broken kernel source for OnePlus Nord (It has broken NFC). So on custom kernels nfc doesn't work and on official builds it is necessary. Also you should check previous posts in threads before asking questions. This same question has been answered a lot of times.