"-vf scale=x,crop=y,expand=x" Does Not Work

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I wanted to generate black pillars on each side of a video, masking some of the
original and not changing the width. The problem is only with widths; a similar
problem does not exist for heights (leterboxing).

The "scale" video filter is not necessary to generate this bug, it is just an
easy way to show the pre-crop width.

Basically, "-vf scale=x,crop=y,expand=z" does not work when x = z. There is no
problem when x != z.

Thank you.