Irregular Watering Patterns (under watering)
Though most Fiddle Leaf Fig Tree parents tend to overwater their plants (keep the soil waterlogged), and give it too little light, this condition is from drought-like conditions, usually coupled with increased light levels.
Fiddle Leaf Figs need evenly moist soil through the Growing Season(March-September). When the plant doesn't have enough access to moisture in the root zone, a condition occurs that affects Ficus lyrata's newest growth.
Reddish, brown spots are created on the bottom sides of the leaves that show through the leaf.
According to the University of Florida Excessive Moisture Stress (which sounds like too much moisture, but is not) is the driver of this condition.
That means the plant is exposed to irregular watering patterns. This also typically occurs in combination with higher light levels.
This creates the perfect storm to expose the irregular watering patterns. Because light and water go hand-in-hand in consumption during photosynthesis, the more light a plant is exposed to the more water it will need. You can learn more about that relationship here.
Lack of water in the root zone makes it challenging for plants to absorb necessary minerals from the soil. Make sure to keep your Fiddle Leaf Fig Tree (Ficus lyrata) soil evenly moist through the growing season, and allow it to dry out slightly more in the fall and winter.
And be sure to give it bright, but indirect light! That is explained here in a 1-minute video.