Unhappy Stromanthe triostar

So… I finally bought myself a Stromanthe triostar, which was a plant that caught my eye and I wished to have one for quite some time. It arrived a little patchy (the brown spots on its leaves were already there when I got it, not sure what they are from, but I checked for pests and root rot and didn't find anything), but overall quite bright and happy looking. A month later, I was so happy to see it started pushing out new leaves. Not even two weeks after, all of the new leaves turned brown, the plant got droopy, dull, and looks like seasonal depression hit it hard. I have no clue what happened to it, after watering the excess water drained nicely, it's roots are still looking healthy… The only thing I can think of is, that I live in a country with dark winters and it maybe didn't get enough light. I'm truly at a loss here, and I'm not sure what to do to help her out, and if she will recover. Any tips and possible explanations would be greatly appreciated.