I would check the watering regime (frequency, amount of water), soil type and the soil moisture (before watering so they are neither too dry or too wet). In the summer, they need more water than in the spring, even though they are drought tolerant. Water deeply when the soil is dry. Skip watering when the soil is already moist or soggy. There is also a rare bacterial disease called oleander leaf scorch that yellows out leaves.
Answered on July 23, 2021
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