I was curious to see, why it is necessary to re-install Magisk after any LineageOS update. So, I downloaded the most recent nightly revision of LineageOS for my Galaxy S9 and had a look inside.
Code:
Archive: lineage-17.1-20201130-nightly-starlte-signed.zip
signed by SignApk
Length Date Time Name
--------- ---------- ----- ----
322 2009-01-01 00:00 META-INF/com/android/metadata
544843564 2009-01-01 00:00 system.new.dat.br
0 2009-01-01 00:00 system.patch.dat
2097304 2009-01-01 00:00 META-INF/com/google/android/update-binary
2205 2009-01-01 00:00 META-INF/com/google/android/updater-script
32337936 2009-01-01 00:00 boot.img
1028 2009-01-01 00:00 install/bin/backuptool.functions
2233 2009-01-01 00:00 install/bin/backuptool.sh
7364 2009-01-01 00:00 system.transfer.list
1342 2009-01-01 00:00 META-INF/com/android/otacert
--------- -------
579293298 10 files
I think, the boot.img included with the zip package explains, why it is necessary to re-install Magisk. Since Magisk tweaks the boot.img it has to be (re)-installed after the installation of a new LineageOS update.