Not sure if this is better as bug report or feature request . . .

Spine alignment options do not work for text in Slug Area. For example, text set to Align Towards Spine and positioned in the slug area beside a recto page does not align toward spine.

It makes sense why spine alignment for objects generally might not work for text not touching a page, since the pasteboard does not have a spine, but the slug area should be recognized as an extension of the page (or, at least, the requirements of the page).