When adding new blank page and specify to use an existing tab, the tab specified to re-use doesn't get added as the list of pages associated with that tab (Setting: Tab Also Current for Pages in app builder: 4000:4305).
I tested adding a new page type as SQL report, and the page value successfully gets added to the setting Tab Also Current For Pages. So without testing each page type, presumably it's just blank pages.
Seems to affect both one and two level tabs.
Thanks for bringing this to our attention. The issue does appear to be specific to the creation of blank pages. I've logged bug 16729276 to track this issue. In the meantime, the workaround, which I'm sure you've already done, is to edit the page and update the tab to reference the new blank page ID.