Whenever I go inside the skill and say one completely random word, the Fallback Intent is not triggered. The echo will just emit a sound and in the Alexa simulator, it would just show nothing. But I know for a fact that I am still inside the skill and the session has not yet ended since if I try to say an utterance that is mapped to a certain intent without including the word Alexa, it would respond correctly. BUT, if I try to say TWO completely random words the Fallback Intent is triggered. For example(this is already inside the skill), if I say the word "pizza" it would just respond with that weird noise and stay in the current session. But if I say the words "pizza pie" it would map to the Fallback Intent.
I have observed this behavior in a skill that has many custom intents each having many utterances configured. But when I tried inputting the word "pizza" to a skill with only 3 custom intents, the Fallback intent works fine.