There's also a yearlong gap between Synthesis and Seize the Fire, so there would've been plenty of room for Tuvok to receive and deal with the news "off-camera" between installments.
That's good. Janeway's death had been mentioned as paart of Tuvok's continuing mental descend, so I thought the news would mean a tremendous change for him. Maybe this will be addressed in more detail in the next Titan
Hopefully the next Titan book will be by a good character writer