It might be the other way around, as in too much water, not enough sunlight.

These trees like to be right up in front of windows, so the sun shines through directly on to them. And for the water, they like to dry right out, so it feels like there's no moisture around the trunk or deep in the bottom of the pot, and then be watered thoroughly.

Oh yes, I know! Lots of the design type blogs say pretty much the opposite care. Some advice does depend on your type of soil and whether your pot has drainage holes.

The advice here comes from people with real-life years of experience, and if you browse the threads about browning FLF leaves (a common problem) you'll find pretty consistent recommendations. Explanations too, which I think is the important part.

More light will help, the only thing to watch inside is if the spot in front of a window gets too hot. FLFs can live outside in full sun too, but you have to acclimatise them gradually to that, over a couple weeks.

Agreed. Too much water / a soil with too many small particles.

Full sun loving trees will never do well in a dark corner.

It's like the root performance decreasing continuously. Maybe some of the root section dead already. Under low light/temp, that tree got difficulties on removing excess water through transpiration, so high porosity soil required.