Hi Haffy,
I care for more than a dozen Ficus lyratas in a variety of indoor home and office locations in NYC. All of then are exposed to very dry air throughout the winter months and that is not an issue as long as they are watered properly.
You described the light as being bright indirect and in my experience most people overestimate light intensity. Lyratas must be right in front of a north or east facing, completely uncovered window. They can be off a bit to the side or a few feet back from a west or south window. I suspect that inadequate light is the primary cause of the leaf drop that yours is experiencing. That would also explain its staying moist for a long time because light determines how much a plant grows and how much water it requires.
Improve the light, but be patient as it will take a month or two for your Lyrata to recover. Plants tend to react slowly to changes in their environment.
You didn't mention repotting, so I will assume you have not done that. If you did, then that may very well be contributing to the problem.
~Will Creed, Interiorscaper
