[OnePlus 7 Pro Guides, News, & Discussion] larsdennert:
If you have managed to root your phone with Magisk, getting updates after that for the stock ROM isn't always a smooth thing. Here are some steps to be successful with updates.

1. Open Magisk Manager and disable all installed modules by unchecking the circle for each.

Please note that steps 2-4 can be done with the stock 1+ updater so long as it senses that you are rooted and offers to download a full rom. So a note about magisk hide which is used to hide root. Make sure you haven't managed to hide root for any process related to the 1+updater or it won't know you are rooted and will offer incremental updates.

2. Install Oxygen Updater (This app may become abandoned in 2019 requiring the use of the stock updater which is slower or manually downloading roms.) https://play.google.com/store/apps/details?id=com.arjanvlek.oxygenupdater

3. Open the app, verify that your device is identified correctly. Open it's settings and set Update Method to "Full Update"

4. Check for an update and download it. Make sure you end up with a full 2GB ROM image not an incremental small one. It will be in the root of internal storage.

DO NOT use Oxygen Updater to update the phone (unless you have TWRP and that is for a different GUIDE)

5. Open the built in OnePlus updater: Settings - System - System Updates - Gear - Local Upgrade and select the newly downloaded update. TAKE NOTE while it is installing which slot it is installing too. It will say something like "inactive slot a". Remember this as it is the "new" slot.

6. After it has installed DO NOT reboot the phone or you will lose root. Open Magisk Manager and install Magisk, even though it says "installed". Select the option when it prompts for "after OTA". This will install magisk in the boot partition of the new slot.

7. Reboot and If all went well you shouldn't boot loop and you'll be running the new slot. If you want TWRP (optional), install the zip now from Magisk->Modules->+ (sign) then select the TWRP zip file and install it. Do NOT reboot. Then do a "Direct Install" of Magisk again, then reboot.

8. Open Magisk Manager and go to the downloads section and reinstall the modules you want to use. Do this from the Downloads screen or using the + button on the Modules screen. This will install them in the current slot.

DO NOT simply reenable the modules with the check mark or you will boot loop. Some modules aren't listed in Magisk and can't be automatically moved from slot to slot. If you installed one of those previously, you must reinstall it with the plus button manually from the modules tab.

You should be done now.

Oh no I'm boot looping: Don't panic.

1. Make sure you have adb/fastboot 1.4.3 or newer on a PC https://xdaforums.com/showthread.php?t=2317790

2. Boot the phone into Bootloader/fastboot either through the recovery menu or VolDn + Pwr

3. If in step 5 above the inactive slot was "a", issue the command from a PC command prompt:

fastboot --set_active=b

or vice versa

fastboot --set_active=a

If you are not sure which to use try one or fastboot getvar current-slot

4. Then: fastboot reboot

This will allow the phone to boot into the last good ROM version. Now you figure out what went wrong.

Another option If your phone didn't retain root, you can stay in the unrooted slot. download a rooted boot image of your firmware version, place it in the your fastboot pc directory, reboot the phone to fastboot and issue the command: fastboot boot boot.img (or whatever the downloaded image is called.) this is a temporary root that is non destructive. Then do a direct install from Magisk manager even though it already says installed. That will permanently root your boot copy. Then install your modules.

Here is the 9.5.11 1917AA rooted boot image https://www.dropbox.com/s/6csxd9zaa4mihwk/boot_a_backup_2019-08-04_19-05-29.img?dl=0

If your phone looped right after flashing, you may not have disabled Magisk modules properly. You can still disable them from the good slot, swap active slots again using fastboot and see if it boots into the new ROM. If not, swap back to the good old slot again and start over flashing again from the beginning.

If you looped after reenabling your modules, it is possible you only enabled one but didn't install it into the new slot. Again swap back to the good slot and disable the modules. Swap back to the new slot and install the modules again even if Magisk thinks they are there.

If all else fails, you can stay in the good old slot until you sort things. If you are looping in both slots then something really went bad and you have to flash back a full ROM from fastboot. See this thread

https://buzzfire.de/xda/t3931424

Then root again using the rooted boot image above.

You may be able to boot into the old rooted slot and root the new slot with the ota option also.

If you have issues where stuff like Wifi doesn't work, then somewhere along the line or previously you flashed a (boot) image that didn't match the /system image in the slot you were working on. This error will persist in that slot every time you use that slot until it gets a fixed set of images.

Hope this helps!