I updated both my S8 and my wife's S8 to the new security update (G950USQU5CRG3) with any issues, so at least for me the update was safe.
This has seemingly bricked my phone. Im stuck in a boot loop and Im getting an error when I enter recovery mode. DO NOT UPDATE!
I managed to fix it with these steps for anyone who passes by.
1. Download this Firmware update (the last firmware update from a few months ago)
2. Download Odin
3. Extract files and start Odin
4. Place each file into their respective slot from the extracted files and be sure to put home_csc into the csc slot.
5. Start phone in download mode by pressing Volume down and Bisby button at same time during boot.
6. Once you are in download mode press volume up to go into download mode menu. Odin should show your phone has connected so press start
7. Allow process to complete and phone will reboot.
8. If error occurs again (mine did at 32%) booted into recovery mode and wiped the cache file.
9. Reboot phone and phone should boot properly.
I'm not seeing any issues like this being reported internally. Might be something that went a little wonky with your phone. I don't imagine the same thing would happen on a replacement device.