I have the same problem and it it is solved with changing slots from default to alternatve (A to B or B to A). Than change it back to default and try to instal TWRP. It worksd for me and now I can install Magisk and whathewer from internal memory.
I hope this will help.
I tried also that changing the slot A / B and nothing worked. Still faced the issue.
So for now I did the boot img method and wait to see if a fix will come or not.
Anyway thanks for your help. ;-)