[Google Pixel 2 XL Questions & Answers] cognitivedissonance:
I'm about 4 hours of reading in on this matter, and I still don't have a proper answer. I've run through countless threads with similar titles, and found some helpful tidbits, but nothing the resolves or fully explains what is going on here.

I just made the jump from Android 9.0.0 (latest) rooted to Android 10.0.0 (latest.) Here were my steps:

Download the latest Platform Tools & unzip

Download the latest Android 10.0.0 Factory image for Pixel 2 XL & unzip into the platform tools folder

Download the latest USB drivers & install

Hook up my phone to the computer (Windows 10) and verify that ADB & Fastboot are working properly via command prompt

Execute the flash-all.bat (left the -w flag in so that it wiped my data) -- it appeared to be only flashing the one slot except for a couple mentions of the other slot's system towards the end of the commands

The phone booted just fine and I went through and installed TWRP and Magisk 23 and so on

Here's where I encountered the error:

As I was rebooting out of TWRP, I realized I'd never used the "Slot A" or "Slot B" buttons, so I picked one, and it started rebooting the phone. The normal screen with the yellow text about phone being unlocked came up, then switched to the white background with the 'G' as expected. The expected next step would've been the 'G' spinning into a circle for a second, and then going to the homescreen, but instead what happened was the screen went black, and then a screen that said "Your device is corrupt. It can't be trusted and may not work properly." showed. It looked similar to the one when the phone first boots with the yellow text about the bootloader, but this text was red. I attempted to use the power button to bypass it, and when I did, it switched the the normal yellow-text-about-bootloader screen, and then switched to the 'G' with the white background again, but there it stayed, just frozen.

I was still able to boot to bootloader by using the power + down button, and could then boot to the stock recovery, but that was it. After a bit of reading, I discovered the following command:

fastboot set_active a

or

fastboot set_active b

and also fastboot getvar all

The set_active command chooses which Slot is being used, and the getvar command shows all the fastboot variables, one of which shows the "current_slot." Once I set the other Slot to be the active one and rebooted, the phone booted normally. Buuut, now I'm concerned that someone is screwed up with my phone, and it's going to fail at some point when I don't have my computer handy, so I went back to work trying to resolve the error when I booted out of the one slot.

Here's what I've tried:

Run the flash-all.bat file in one slot, let the phone reboot, boot to bootloader, switch slots, reboot to bootloader, flash-all.bat in the other slot.

Same as above, but without the -w flag.

Manually flash the bootloader, radio and image zip on both slots

Install the TWRP while booted into the adb-loaded TWRP img (this seems to write to both slots based on the output)

Install and uninstall Magisk both via recovery and via the patched stock boot image.

After every single attempt, the one slot would work just fine, and the other slot would exhibit the behavior I mentioned. Depending on which Slot is flashed first, the Slot that fails to boot properly does change (ie. it's not always Slot A or Slot B that fails.)

Untested methods:

Screwing with dm-verity -- I saw one thread that gave a few fastboot commands to disable dm-verity, and a couple people reporting success with it. After searching around and doing a bit of reading on dm-verity, I still don't understand what it does, whether disabling it would have any negative repercussions, and whether disabling it would fix the issue legitimately, or just kinda mute it.

Using Deuce's bootloop recovery tool -- again, I don't really know what that thing is running, it hasn't been maintained in quite some time as far as I can tell, and there are several threads indicating that it runs a bunch of haphazard stuff that could cause more problems. Some reports indicated it worked for them, but I'm concerned about what else it might be doing that may negatively affect things down the road.

Re-locking the bootloader -- I saw this one mentioned several times, but I also saw several people warning that you could brick your phone if you do it wrong. I also saw one or two folks who seemed like they couldn't get back to being unlocked after re-locking. I don't want to screw with this option given those risks.

Can anyone give me any more tips on how to resolve this? I'm pretty sure that the Pixels are supposed to run equally well from both slots, so I'd really like to get the matter resolved so I don't have to worry about what would happen if my phone moved me over to the other Slot for some reason. Any insight and explanations are appreciated!