Fiora Victorious became mighty because she was buffed from a stun during a combat. Does she get the shield +1 as well now that she is mighty, or has that trigger been missed?

To answer your question: Yes, Fiora, Victorious automatically gains the Shield bonus the moment she becomes Mighty.

There is no "trigger" for this that you can miss. Because Shield is a passive ability (Rule 814.1), it is always active as long as the condition for the ability is met.

Here is how the interaction works:

1. Passive Nature of Shield: Fiora, Victorious has the text: "While I'm [Mighty], I have [Deflect], [Ganking], and [Shield]." According to Rule 814, Shield is a passive keyword that functions as: "While I am a defender, I have +1 [M]."
2. Layers of Interaction: When Fiora receives a buff (such as from a stun interaction) that brings her to 5 or more Might, she satisfies the "Mighty" condition. The game automatically updates her characteristics in the "Ability-Altering Effects" layer (Rule 471).
3. Automatic Application: As soon as she becomes Mighty, she gains the Shield keyword. Because she is currently in combat as a defender, the Shield keyword immediately grants her +1 Might. This is not a triggered ability that goes on the chain; it is a static game state update.

Summary: You cannot "miss" this bonus. As soon as her total Might hits the 5 threshold, she becomes Mighty, gains the Shield keyword, and immediately receives the +1 Might bonus from Shield, bringing her to a total of 6 Might.