in order to fix this install OOS12 and flash LineageOS 20 on top of it using the steps mentioned in LineageOS wiki.
In order to revert to OOS use msm tool (following steps):
- Download the correct MSM tool for your device and region from here: » onepluscommunityserver Unbrick Tools
- Download and install Qualcomm EDL driver from here: » Mediafire Qualcomm driver
- Follow the steps in this XDA thread in exact order
, it is also important to wait the correct amount of seconds etc., don't download the software from the XDA thread use the one downloaded earlier instead » XDA Restore your device to OxygenOS
Afterwards install OTA updates until you are greeted with OOS12, then install LineageOS.