My ZZ plant has been doing great, and I was happy to see that five new stems shot up recently. However, today I noticed that one of the stems has a black spot on the unfurled leaf. I do not have my plant in bright light. It's near the middle of the room where the light is quite low. I try not to water too often - maybe once every few weeks. Google tells me it may be root rot, but this would seem odd considering how well the plant otherwise seems to be doing. Any insights are much appreciated.