[Nokia 8 Questions & Answers] boteeka:
Here is what I have, Nokia 8 TA-1004:

- on slot A I have Android 8.1.0 with October security patch

- on slot B I have Android 8.1.0 with September security patch

Neither of these would apply the OTA (slot A November patch, slot B October patch).

I got rid of previous root attempts by flashing:

- stock October boot image to slot A

- stock September boot image to slot B

At this point I am back to as stock as possible.

Then I tried to follow your suggestions, flashed:

- prerooted October boot image to slot A

- stock September boot image to slot B

Set the active slot to A.

Reboot.

Install Magisk from MagiskManager to the "other slot", this switches the active slot to B.

Reboot.

Check that Magisk is installed: yes. Check root with RootChecker: there is root.

Uninstall Magisk from MagiskManager with the Restore Images option, this should restore the stock boot image on the active slot, which at this point is B.

Without reboot, try to apply the OTA update to upgrade to November security patch.

Update fails. Here are a few relevant lines from the log (retrieved with adb logcat):

11-27 16:56:59.961 1107 1107 I update_engine: [1127/165659:INFO:delta_performer.cc(368)] Applying 5 operations to partition "abl"
11-27 16:56:59.965 1107 1107 I update_engine: [1127/165659:INFO:delta_performer.cc(661)] Starting to apply update payload operations
11-27 16:56:59.970 1107 1107 E update_engine: [1127/165659:ERROR:delta_performer.cc(1065)] The hash of the source data on disk for this operation doesn't match the expected value. This could mean that
the delta update payload was targeted for another version, or that the source partition was modified after it was installed, for example, by mounting a filesystem.
11-27 16:56:59.970 1107 1107 E update_engine: [1127/165659:ERROR:delta_performer.cc(1070)] Expected: sha256|hex = 1376DA9AFC602B63352F6611917129A8FEC841070A5A44B657826B25D3BAED0D
11-27 16:56:59.970 1107 1107 E update_engine: [1127/165659:ERROR:delta_performer.cc(1073)] Calculated: sha256|hex = 658502232C3D562E1883E66BB2C4BED4F48E98CB02B983DC43DB6D9B05ABF939
11-27 16:56:59.970 1107 1107 E update_engine: [1127/165659:ERROR:delta_performer.cc(1084)] Operation source (offset:size) in blocks: 1:1,3:1,36:14,51:16
11-27 16:56:59.970 1107 1107 E update_engine: [1127/165659:ERROR:delta_performer.cc(1250)] ValidateSourceHash(source_hasher.raw_hash(), operation, error) failed.
11-27 16:56:59.970 1107 1107 E update_engine: [1127/165659:ERROR:delta_performer.cc(288)] Failed to perform SOURCE_BSDIFF operation 1, which is the operation 1 in partition "abl"
11-27 16:56:59.970 1107 1107 E update_engine: [1127/165659:ERROR:download_action.cc(325)] Error ErrorCode::kDownloadStateInitializationError (20) in DeltaPerformer's Write method when processing the received payload -- Terminating processing

Then I went back to stock boot images again, and tried the whole process the other way around: switching slots.

Stock image on slot A.

Prerooted image on slot B.

The rest of the process is the same as above.

This time I get a different error in the log:

11-27 17:07:30.788 1050 1050 I update_engine: [1127/170730:INFO:delta_performer.cc(368)] Applying 5 operations to partition "abl"
11-27 17:07:30.794 1050 1050 I update_engine: [1127/170730:INFO:delta_performer.cc(661)] Starting to apply update payload operations
11-27 17:07:30.834 1050 1050 I update_engine: [1127/170730:INFO:delta_performer.cc(368)] Applying 22 operations to partition "bluetooth"
11-27 17:07:30.841 1050 1050 E update_engine: [1127/170730:ERROR:delta_performer.cc(1065)] The hash of the source data on disk for this operation doesn't match the expected value. This could mean that
the delta update payload was targeted for another version, or that the source partition was modified after it was installed, for example, by mounting a filesystem.
11-27 17:07:30.841 1050 1050 E update_engine: [1127/170730:ERROR:delta_performer.cc(1070)] Expected: sha256|hex = 73D9DD6F7FCBA3C7A6900599BB66C5D321C9F77B74808FD1ADEB50DD8CE475D2
11-27 17:07:30.841 1050 1050 E update_engine: [1127/170730:ERROR:delta_performer.cc(1073)] Calculated: sha256|hex = BEFE956EC57F320FA6F96067CCDF81DEF88D227A15D7E5412B2064A941E8E7BF
11-27 17:07:30.841 1050 1050 E update_engine: [1127/170730:ERROR:delta_performer.cc(1084)] Operation source (offset:size) in blocks: 4:1,11:1
11-27 17:07:30.841 1050 1050 E update_engine: [1127/170730:ERROR:delta_performer.cc(1250)] ValidateSourceHash(source_hasher.raw_hash(), operation, error) failed.
11-27 17:07:30.841 1050 1050 E update_engine: [1127/170730:ERROR:delta_performer.cc(288)] Failed to perform SOURCE_BSDIFF operation 11, which is the operation 6 in partition "bluetooth"
11-27 17:07:30.841 1050 1050 E update_engine: [1127/170730:ERROR:download_action.cc(325)] Error ErrorCode::kDownloadStateInitializationError (20) in DeltaPerformer's Write method when processing the received payload -- Terminating processing

As you can see in the first case it is complaining about the "abl" partition, in the second case it is complaining about the "bluetooth" partition.

webvan, what is your phone model? Is it TA-1004? What security patch levels were you working with in your case?