Eating watermelon with other fruits is often discouraged due to various reasons here are some.

Watermelon digests faster than most fruits. Eating it with slower-digesting fruits can cause fermentation in the stomach, leading to gas and bloating.

High Water Content: Watermelon has a lot of water, which can dilute digestive enzymes and slow down the digestion of other fruits.

Different Sugar Composition: Watermelon is high in fructose, which can mix poorly with other fruit sugars, potentially causing bloating and gas.

Acidity and Alkalinity: Watermelon is slightly alkaline, while many other fruits are acidic.

Nutrient Absorption: The high water content in watermelon can sometimes prevent other fruits' nutrients from being fully absorbed.

Fruit Mixing Principles: mixing different types of fruits, especially those with high water content, can cause digestive problems.

High Glycemic Index: Watermelon can cause a quick rise in blood sugar levels. Mixing it with other fruits can make this worse.