I am running LineageOS 15.0 and with TWRP Recovery 3.2.0.0, I did a full system backup with the new updated recovery a couple days ago and the backup finished without any error. It's weekend, I think I have a little time to play around, so today I tested to run a full system restore with my backup but the TWRP Recovery restore process just failed (keep on running without progress and didn't give any error message). Eventually, I have to fastboot and flash back old TWRP Recovery 3.1.1.0, with this old recovery, the restore process went by smoothly, so I concluded that there is nothing wrong with my previous backup but rather a problem of the new TWRP Recovery 3.2.0.0 or the new recovery conflicts with Oreo. So if you are running TWRP Recovery 3.2.0.0, i think you would better double check to confirm your full system backup is good to go.