Dracula leaves yellowing from the top down - sunburn?

[Zyzzyzus]

Hello,

I have three clumps of Dracula lotax in a terrarium. It seems like the leaves are slowly turning yellow from the top down, and then dropping. I know Dracula's are finicky, but this one is supposed to be easier. All three clumps are pouching out a lot of new growth, too. I've tried searching to see what yellow leaves can mean and haven't found anything conclusive. Is the leaf yellowing a sign that they're getting too much light? Or too much water? Or...

Thanks!