Okay, I finally figured this out on my own. I studied the get_field_object() function more deeply, and on this documentation page, in the “field_key vs field_name” section, there is a hint. It is not said directly, but apparently even if posts have been created that use the Field Group, if a filter causes the current query of posts (using the pre_get_posts hook) to return none of them, field_name won’t work. Even Elliot used the field_name in his tutorial, but in the case of a filter like this, you MUST use the field_key, even though it makes the code harder to read.
