SQL Server 2019 using Linked server to Oracle 19c (19.9) using Oracle 19c client on Windows - DBCOLUMNFLAGS_ISNULLABLE error
I found a support.oracle.com Doc ID 1519958.1 that describes the issue.
For solution it says: Use the relatively simple work around of linking to a view that is based upon the table, instead of directly to the table.
Apparently this issue has been around since 2012.