Too bad, I just had a little of hope I did something wrong on my side.
> The most probable cause of the mods failing is because the Android 10 and 11 update could have used an entirely different format as opposed to the one from Android 9, and so the same scatter files won't work.
Does fastboot have anything in common with scatter files and SP Flash Tool? I don't think so, but I may be wrong as its my first phone I did flashing / modding on. As stock boot flashes fine via fastboot, I would rather say either Magisk produces the incompatible format of the boot partition (I don't know how to properly unpack / mount boot partition and check the changes, so I have no way to debug it further), as the boot image I got when doing readback on Android 11 fine when flashed back with fastboot (and I'm pretty sure it wouldn't work). Anyway, I personally believe something's blocking us from booting from other partitions after Android 11 update, probably by checking its checksum – but I may be wrong there.
The other thing is that I can't boot back to Android 9 after an update when changing the active slot to A – where A partition scheme shouldn't be touched at all when updating to make it possible to recover when update failed to flash correctly (at least from my understanding of A/B partition scheme). This could be the same case as of custom ROMs, where something else needs to be tweaked so boot verification isn't checked. Anyway, that's my opinion and I might be wrong, I'm complete novice in flashing, yet I have a basic understanding how most common OSes works.
As of Android 11, there's a little hope about being able to boot into any GSI image via DSU Loader (it will work without the need of flashing anything and maybe without the need of having the bootloader unlocked) – I still haven't checked if it is possible. If you're curious about my progress, just call me back and I'll be glad to share with you if that works. I'll use the method described in Android Developer website:
Dynamic System Updates (DSU) lets developers install a GSI side by side with the device's system image on a DSU-supported device that runs Android 10 or higher.
developer.android.com
Anyway, I really appreciate you answered me, I'm glad at some point I didn't messed up anything and that's the current limitation everyone needs to deal with.