Hello urg

I planted 2 Escallonia "Donard Radiance" shrubs last July (South-facing
border, slightly acidic clay soil, less than ideal drainage). I did
notice at the time that there were a few yellow leaves on one of them,
but didn't worry about it too much.

The plants have since grown quite vigorously and seem to be generally
healthy. However, about 10% of leaves on both plants are now yellow or
yellow/green (these are randomly distributed). As these leaves can be
easily shaken off, I'm assuming that this is indicative of some sort of
a problem and is not a feature of the plant.

For what it's worth, the plants were originally planted using a 50/50
mix of top soil/Multipurpose compost, with some fish, blood and bonemeal
thrown in. I fed the plants with Miracle-Gro "Shake and Feed" slow
release all-purpose plant food in early March of this year, at the
recommended rate.

Is this something I ought to be worried about? If so, what should I do
about it?

Many thanks

Paul K

PS Just in case it's not obvious, I'm a novice gardener!