Posted this to bugs.kde.org. Just copy/pasting what I said there.
In dolphintabpage.cpp:
Through the connections set up in createViewContainer(), m_primaryViewContainer->setActive(true), at line 98, ends up triggering slotViewActivated().
Because m_primaryViewActive is true, oldActiveView is not set to the view needed in this particular situation. Both oldActiveView and newActiveView end up pointing to the same view and the connections are not set up.
Adding
m_primaryViewActive = false;
inside the if clause at line 93 seems to fix it?