Nepenthes, or the Asian pitcher plant, naturally grows in the tropical regions of Asia either in high mountain rain forests or in warm lowland grasslands at the base of mountains. They are divided into two types, those that live above 3000 feet (highlands), and those that live below that (lowlands) with differing requirements. The Nepenthes x 'Miranda' is an intermediate or lowlands type and requires warm night time temperatures (above 21 deg C). Below this, and they are subject to developing brown spots on their leaves.
Lowlands Nepenthes requires a high humidity level and so you might be tempted to grow them inside a terrarium. But they rapidly outgrow most household terrariums and then you have the problem of trying to get your large plant to adjust to lower humidity levels.
I note also that you've repotted your plant so it may be suffering some sort of transplant shock. Did you use exactly the same media as the plant came in, or something else?
Nepenthes also requires high light levels and a 20 Watt terrarium light is insufficient. You'll need at least 2 x 40 Watt fluorescent lamps 8 inches above the plant, kept on for 14 daylight hours during Summer and Spring, and 12 hours for the rest of the year.
To see whether the light from a window might be sufficient, you can install a lux meter app on your phone and measure the light from the window and from your existing lights to see if they're anywhere close.
Since they live at the base of mountains, their roots are bathed in oxygenated water running down the mountains. So, you need to keep the roots moist but not let them sit in stagnant water. If you let the soil dry out, you will kill the plant. People have even grown them successfully hydroponically.
It's not normally needed to feed these plants since they require only minuscule amounts of nutrition, eg. one insect a month. The roots do not take up any nutrients of note. As an alternative to insects, you can feed the pitcher with 1/4 strength orchid fertilizer once a fortnight.
Based on the above, I suspect your plant is being underwatered and has insufficient lighting.
https://www.growcarnivorousplants.com/Articles.asp?ID=266Nepenthes Care Guide