How i did it was.
Folloe the same procedure for root again and in magisk allow retention script.
It's in magisk apk to keep root after update. You can find that in oneplus 6 guide page.
The 23 and 25 Dec builds have this issue. If you can find a older version of PE it won't happen.
I tried Syberia and it's also buggy. Install HavocOS it's the best option for OnePlus 6.
I faced the same error, open the zip using winrar or alternatives edit the metadata file in META-INF\com\android folder, edit the last line to pre-device=enchilada