overwater or less water, What is the difference

I have the same problem with my Lagerstroemia indica in the begining of summer and at summer , the leaf turn the form and change from green to yellow.
I have very care to not overwater , but i´m not sure if this problem if for the water or for another else.
some pictures
Thanks in advance

My first assumption is lack of fertilizer? Especially lack of magnesium often shows as yellow leaves.
The leaves also looks like they have had too much sun/lack of watering, because they look dried out at the tips especially.

Over watering or damaged roots will give the same signs, because the roots then not will be able to take up sufficient water or/and nutrients.

Regards
Morten