Welcome to TFF.
You are not really doing anything wrong. What you see--the lowering of the pH--is perfectly natural. In any aquarium, organics will accumulate. Fish produce these from their food, but organics also occur naturally from other life forms. As the organics accumulate, the breakdown by bacteria produces ammonia and CO2. The CO2 creates carbonic acid in the water, and the pH naturally lowers accordingly.
This can be offset by the carbonate hardness (KH, or Alkalinity) of the water. KH acts as a buffer to prevent pH fluctuation. The stronger/higher the KH, the more buffering capability. With RO water, you have removed all minerals, so the KH along with the GH (general or total hardness, which is primarily the amount of calcium and magnesium) is zero. The organics can thus accumulate and the water will become more acidic.
Wood also adds to the organics, though not all that significantly. The substrate you are using is also contributing to this; from their website:
Helps support neutral to slightly acidic pH - ideal for most plants, tropical fish and shrimp normally kept in planted aquariums.
No mention is made of fish species; those from very soft water habitats will thrive in this environment. Some shrimp should, but other shrimp might need more mineral. There are ways of doing this, but you haven't asked that so I won't get bogged down. The above hopefully explains the biological process that is occurring.
Byron.