When you copy from a source directory with a single file without the trailing slash to the destination path, the file is renamed to the destination directory name. Double check your command and ensure that you append a backslash to the destination path:

gs://bkt-tv-mytest-lnd/mydataase.db/mytable/load_dt=20250902/

This indicates that the destination is a directory, not a file.

You can also explore other options like Storage Transfer Service for large-scale transfer or gsutil for smaller transfers.