Beginner Gardening: Fiddle leaf fig (ficus lyrata) with brown/yellow along veins, 3 by indogirl

indogirl wrote:
I've had my fiddle leaf fig for 2.5 years. We got off to a rocky start with some brown patches on the leaves, but it has since recovered and been doing well. In the last two years it has grown a couple feet and added many new, healthy leaves.

However, I just noticed some disturbing brown/yellow spots forming along the veins of a couple leaves. Most of the leaves are fine, but I've posted pictures of the damaged leaves below. Any idea what this could be? Over watering? Under watering? Too much sun? Too little sun? Disease? I'm at a loss.

I've had it in the same location, same pot, same watering schedule (once/week), etc. The only thing that's changed is the outdoor weather has gotten cooler so we have the heat on inside (set at 70 degrees).

I'd love to hear any of your thoughts or advice. Thanks!