If it is sunburn, it won't "improve"; but new growth should not have the spots. It could also be snails/slugs that eat the top surface of the leaf and the sun then bleaches out the spots. The only remaining explanation would be if you have sprayed some chemical on the leaves when the temperature was too high.

It looks to me like a chemical burn.

Your roots look good and healthy

Where were your tree kept? On a patio? On the ground? In all day full sun? Do the newer leaves do the same thing in a short period of time? Do you spray the leaves with anything but water?

Thank you all for those answers.

I keep my citrus indoor but I've put them one or two week on my patio in the middle of summer.

I don't know if it's sunburn because white spot continue to appears months after I've put my plants indoor and away from the sun. I think it's a chemical burn most likely.

One time, and one time only (hopefully) I used a "water spray" for human face on my citrus to hydrate them.

But the water contain sodium, sulfate, bicarbonate, chlorine, nitrate, silicon dioxide... :(

Yup, chemical burn. Probably the water spray. New leafs should be fine