It may be. What does a good reliable soil test say about the growing conditions around these plants?

Haven't done a soil test. It's heavy clay - seems very rich in nutrients.

Whatever the disease/condition is, it doesn't look like blight. The whole tree looks like yellowish wax. There's two wholly waxy thujas side by side, and then a third one right next to them is a healthy green.

On other thujas, there's waxy new growth only at the top.

Maybe I'll spary with copper all the same, just to be on the safe side?

Please post pictures.

And don't spray anything until you have a firm diagnosis.
"Just in case" remedies can make things worse.

What affects a plants ability to use nutrients from the soil? Soil pH. Nutrient balance in the soil, an excess of any one nutrient can adversely affect a plants ability to use another.
The only way to know any of this is with a good, reliable soil test.