I've created a bookmark in one project where the scale is 1:18,055. I've exported this bookmark to use in another project, but when I import it, it does not match and the scale is at 1:20,182. Is this a known issue? I can't say I've experienced this before.
As far as I can tell Bookmarks zoom the map display to an extent, not a scale. For example, if you are zoomed in to a bookmark and then pin a pane open thereby changing the display area, then apply the bookmark again the map will zoom and scale accordingly.
I'm not sure I understand what you mean by "pin" a pane open. Do you mean like pin the catalog pane open on the right side which would change the extent of the area of the map? I always have the same arc layout on my screen so that shouldn't matter. If I have a layout in the exact area I want and create a bookmark, shouldn't that translate to any other layout? It doesn't seem to be doing that. The only thing I can think of is perhaps the bookmark was made in the map frame view and doing that is different then making it in the layout view.