I think this is expected behavior.  Placeholders aren't a general text templating mechanism to substitute arbitrary components of a query.  As the docs mention:

With most drivers, placeholders can't be used for any element of a statement that would prevent the database server from validating the statement and creating a query execution plan for it. For example:

  "SELECT name, age FROM ?"         # wrong (will probably fail)
  "SELECT name, ?   FROM people"    # wrong (but may not 'fail')
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