Sorry Dan, should have guessed encryption as I've seen it before, for example https://forums.oracle.com/forums/thread.jspa?threadID=2410571.
There are bugs in the Oracle Support database relating to this. For example, ID 871844.1 describes the same issue but with the statement:
import database 'xxx'.'xxx' data from load_buffer with buffer_id 1 add values create slice;
Supposedly fixed in 18.104.22.168 but evidently not for every case. If you have an Oracle Support account I'd log it.