Garden Pests and Diseases: dark discolouration of ficus lyrata leaves, 3 by ondrej

ondrej wrote:
g\'day,

i have acquired two fiddle-leaf figs within the last few months and have them both in pots near a west-facing window. both are growing, but the new leaves that form have a dark discolouration along the veins on the underside which can be seen through the leaf on the upper surface. only the leaves that have formed in my apartment have the discolouration - those formed prior are normal. does anyone know what this might represent?

the photos follow are:
1. upper side of a leaf on plant A
2. under side of leaf on plant A
3. underside of leaf formed prior to my acquisition. plant A
4. underside of leaf on plant B