The forgotten series in The Walking Dead universe resolves a major mystery from Fear the Walking Dead season 6. The season introduced a new threat in the form of a nuclear bomb and a cult leader named Teddy. This storyline allowed the show to explore the lore and set up a nuclear apocalypse for season 7.
However, there were still unanswered questions about the U.S.S. Pennsylvania and its crew. These questions were later answered in the spinoff series, Dead in the Water. The spinoff revealed the events that led to the outbreak on the submarine and clarified the role of certain characters. However, one question about the presence of zombies in handcuffs on the submarine remains unanswered.
Overall, the spinoff series provides a well-fleshed-out backstory for Fear the Walking Dead's mythos.