u/Few-Lingonberry3119

So I've planted theese dragon fruit back in August and originally I've had 8 seedlings but gave 4 away and repotted the other 4 into a bigger pot with a soil thats around 60% dirt and 40% fine sand and over the past few months I've noticed that one of them is starting to become darker and im starting to get concerned, although it seems happy because it didnt have side branches before so I don't think its rot or anything like that

Has something like this happened to you before and if so is this normal?