My guess would be that it's too dry. It would be better off in the ground than in a pot.

It is showing signs of lack of minerals [possibly], so give it a dose of Epsom salts [magnesium sulphate] watered onto the soil. If it stays evergreen where you are then it could just be the old leaves dying and the plant takes back the chlorophyll leaving them yellow mottled.

if you are able to get it into the ground I am sure it will recover its health.

My escallonia always does this. It never seems to do any damage to the health of the plant.

That's not leaf spot. Escallonia has a tendency for its older leaves to go yellow and drop. If the new ones are doing it I'd be more concerned.

Have a look at the roots and check if the shrub is pot bound or of the roots are being attacked by some grub in the soil.