Slice the pork tenderloin into very thin, bite-sized strips and set aside.
In a medium bowl, whisk together the achiote paste, pineapple juice, apple cider vinegar, dried oregano, ground cumin, garlic powder, sea salt, and black pepper until smooth.
Add the pork strips to the marinade, tossing to ensure every piece is thoroughly coated, and let it marinate for at least 15 minutes.
While the pork marinates, prepare the pineapple salsa by combining the diced pineapple, diced red onion, chopped cilantro, and lime juice in a small bowl.
Heat the avocado oil in a large non-stick skillet over medium-high heat.
Add the marinated pork to the hot skillet in a single layer and sear for 3-5 minutes, stirring occasionally, until the pork is cooked through and slightly charred on the edges.
Warm the corn tortillas in a separate dry pan or over a low flame until soft and pliable.
Divide the cooked pork evenly among the three tortillas and top each taco with a generous portion of the fresh pineapple salsa.