Alright, after digging through the source all day, and with the help of @bigbiff, we were able to determine that libziparchive was the source of the problem with the Lineage OTA installation. I found some patches in the android-9.0 tree and backported them to the 8.1 tree and rebuilt TWRP, and OTAs appear to be installing properly now. As always, if you have any issues, please post on the Issues tab.
Some reminders about this version:
- unified TWRP for both SAR and legacy system setups
- For AOSP based ROMs only, especially Oreo+
- MTP may load incorrect drivers in Windows 10, if you have the HTC drivers previously installed, but it should work fine in Linux-based systems
EDIT: Had to re-upload because I realized I missed the patch to include the LineageOS keys so that Zip verification won't fail, and then realized I forgot the decryption fix. Been trying to do too much these past few days - I think it's time for a break...