Often Saga abilities will skip past rounds of priority and will resolve immediately, forcing a move to the next phase. This can even ignore 'full control' mode.

Example: Final chapter ability of "Summon: Esper Maduin" is on the stack. Even with 'full control' mode activated, there is no opportunity to cast instant-speed spells or activate abilities, such as the ability of a Restless Praire to become a creature (even though doing so in response would be too late to target it with Maduin's ability).

Other times this has occurred casting a Saga in second main phase, and after the ability resolves the game skips straight to the end phase.

Summon: Fenrir perhaps does not shuffle the library after it's first chapter ability resolves. This is visible to players as, when the 'search library' function begins, the top card of the library becomes visible (as now the library in it's current order is known to them). After search is complete, the next card drawn is still the visible card. (Obviously this could be sheer luck... except this has happened to me multiple times in the past couple of days, with cards that that are specifically 1-ofs in the deck. That seems unlikely)