[OnePlus 9 Pro] camparks:
I am able to get L1 to hold with unlocked bootloader and root on OOS A13, converted to Global, but with one caveat.

All my in-depth testing has been on converting to LE2125 Global, because with EU I couldn't ever get past the dual-SIM bug knocking out my data on stock, so I had to give up and go to Global instead. Trying the data fix module did not work either - this was on Verizon, others may have better luck.

My attempts on Global center around information found in Steve0007's root thread, where it says:

Going off the above, you need to flash a payload.bin to keep L1 active when going from A12 to A13. If you flash an update instead, you will drop to L3 - either immediately upon upgrade if the bootloader is already unlocked, or as soon as you unlock the bootloader on A13, or sometimes even drop to L3 on just a reboot despite the bootloader still locked. I assume the same would be true for EU, but I never got to testing since I couldn't get my SIM/cell service to ever work.

The caveat with Global is that you are stuck on whatever A13 you flash payload.bin from. OTA's fail to install, both within the system and using the update apk from OnePlus(looks like what you see in the system on A11). This rang true for trying with both F.16 and F.17, and with OTA's to both F.18 and F.20.

So, here is a condensed version on how to get A13 with L1, unlocked, rooted, but can't update:

Convert LE2127 to EU or Global using craznazn's method.

Either OTA or local install with flashable zips to A12 - for global it should go update once on A11 to 11.2.10.10, then to C.48 for A12.

If you haven't yet, you must unlock the bootloader while still on A12 for the next step. (Side note - when testing taking OTA's all the way, I found that if you haven't unlocked your bootloader yet but plan to later, you MUST at least make sure that OEM unlocking is enabled. If not, it will stay greyed and locked once you go to A13 and the adb shell command won't enable it either)

Use FlashBoot Enhance to flash payload.bin from an A13 flashable zip - I tried both F.16 and F.17, but both have F.18 & F.20 updates fail. Follow the instructions in this post from that thread, plus you will most likely have to delete any prior installation temporary partitions ending in '.cow' - scroll down a few more posts in that thread and you will see some conversation on those. You can't delete those with your bootloader locked.

At this point, you will have A13 with L1, but SafetyNet will be failing on CTS due to the unlocked bootloader. Follow the root directions back in Steve0007's root thread to temp boot a patched bootloader, patch the bootloader on the phone/create a backup, install Universal SafetyNet & Shamiko, Zygisk, Deny List, etc.

In terms of trying to work around the update issue, here is what I tried and didn't work:

Flashing the OTA's locally by downloading from Oxygen Updater still fail.

Flashing the same update as used with FastBoot Enhance locally will work, BUT with the bootloader unlocked will result in Widevine dropping to L3 just like if OTA was done for all updates. I think I tried an update to F.20 after this and it worked, but was still L3.

Flashing both slots via FastBoot Enhance had no effect on OTA's, they still fail.

Flashed F.11 abl.img, but updates still fail.

Edit 02/27/23: Added more info the update attempts