Can not select _PARTIONTIME in SELECT but can select it in INSERT

Question

When running the below SELECT I get this error message

Error: Invalid field name "_PARTITIONTIME". Field names are not allowed to start with the (case-insensitive) prefixes _PARTITION, TABLE, FILE and _ROW_TIMESTAMP

SELECT
    _PARTITIONTIME,
    jobId
FROM
    `project.dataset.audit`
WHERE
    _PARTITIONTIME >= TIMESTAMP("2019-02-20")

However, when I use it in a DML the query is working

INSERT INTO
  `project.dataset.audit_clustered`
  (
      _PARTITIONTIME,
      jobId,
  )
SELECT
    _PARTITIONTIME,
    jobId
FROM
    `project.dataset.audit`
WHERE
    _PARTITIONTIME >= TIMESTAMP("2019-02-20")

I have 2 questions:

1. Why the SELECT alone is not working
2. Is it guarantee that the INSERT will work properly and data will be inserted to the correct partition of the target table

Answer 1

Replace

SELECT  _PARTITIONTIME

With

SELECT  _PARTITIONTIME AS something

This because the resulting set can't have a column starting with _, but you can name it anything else.