Actually the reason for the notice ' verification problem' is one or more of the follows:
1. phone is running a modified boot image patched by magisk which has removed avb 1.0 during patch.
2. Boot loader has been unlocked.
3. Image signing hashes could not be verified.
4. One or more partitions may have been changed or modified eg stock recovery(if running a custom recovery).
Solution:
Backup data partition with twrp.
Unroot with magisk uninstaller. Magisk would restore stock boot.IMG to partition when you unroot.
Reboot to system.
Decryption would be unsuccessful as stock boot image has been flashed while data partition looses integrity check due to missing encryption keys.
"Decryption Unsuccessful"
So accept this and continue with device erase. Device would take 15 to 20 mins to startup and would replace the custom recovery with stock recovery and reencrypt the data partition upon first boot.
Now you can pull up the system update utility and successfully update the phone to a new build number.
Phone would restart to apply the OTA update.
If you choose to root again then tap on build number repeatedly.
Enable Developer Option
check to enable USB debugging and reboot to fastboot.
Flash custom twrp and reboot to twrp.
Format data partition as usual. Type yes and accept.
Flash the encryption patch.
Flash magisk which would remove avb 1.0 from the stock boot.img
Now you can restore the data partition you backed up earlier in twrp and reboot to system without showing the Decryption notice.