In my experiments from last week I was unable to reproduce this.
But here an example of two different — not time-dependent — types of missing value URLs:
This is the result of set tabList to tabs of windows in Safari, after having closed a window with 3 tabs (the only window) and then immediately reopened it via Safari’s “Reopen Last Closed Window”:
Tabs 1, 2 and 3 were the tabs that were open before closing the window. After “Reopen Last Closed Window” all 3 tabs were visibly restored but apparently only tab 1 (the frontmost tab) has fully loaded, hence the missing value for tabs 2 and 3. This is understandable.
Tab 4 was a tab I had closed shortly before closing the window. You see that in the tab list now it appears again, and as member of a different window (id 6959) and with missing value. Although “visible” of this tab is true, note that the parent window of this tab (and hence the tab) is not on screen (= not visible).
Tab 5 is similar to tab 4 in the sense that it also belongs to a separate (and invisible) window (id 7739). However, in contrast to tab 4, I cannot explain where it comes from. It seems like a ghost clone of tab 1.
Here the same thing on window level (set winList to windows):
“document” of the invisible windows (id 6959 and 7739, containing tabs 4 and 5) is missing value, and “visible” of these windows is false.