I guess because although we [set isPrintSelectionRBEnabled](https://searchfox.org/mozilla-central/rev/919607a3610222099fbfb0113c98b77888ebcbfb/toolkit/components/printing/content/print.js#618-630), that setting isn't on the list of propagated settings.
Bug 1670027 Comment 1 Edit History
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I guess because although [we set isPrintSelectionRBEnabled](https://searchfox.org/mozilla-central/rev/919607a3610222099fbfb0113c98b77888ebcbfb/toolkit/components/printing/content/print.js#618-630), that setting isn't on the list of propagated settings.