I don't know if the problem is in the kernel or magisk. If it's magisk, 18.2 is the problem cause its not officially stable. I have a temporary solution for this.
This is what happened when clean install any other rom with stock rom kernel.
Magisk will not install even after clean install or you install manually with any other version. Somehow this problem carried over to others rom or kernel.
If you try to install magisk from the installer, it will say unable to unpack boot image. If you install using twrp, if will say magisk not install also. To work around this, I'm using magisk twrp uninstaller, because the magisk manager doesn't have one appeared. After you uninstall, magisk will still visible but with recent one you install for me its 18.1, the current stable one. After this all works well. Root permission also fixed.
But this problem will continued if you install other kernel or rom. I hope OP can take a look at this problem.
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