This is really a bug in Reader Mode. (You can see the same issue if you click the Reader Mode icon at the right edge of the URLbar for the page in question [looks like a sheet of paper].) (The print preview Simplify Page feature just uses "reader mode" to determine what content to show.)
Bug 1656576 Comment 4 Edit History
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This is really a bug in Reader Mode. (You can see the same issue if you click the Reader Mode icon at the right edge of the URLbar for the page in question [the icon looks like a sheet of paper].) (The print preview Simplify Page feature just uses "reader mode" to determine what content to show.)