Many of the following vegetables give off their sweet taste after being cooked, as they contain significantly measurable concentrations of simple sugars such as fructose, glucose, and the disaccharide sucrose (commonly called white sugar), sugars that are present in higher concentrations in sweet fruits.

For example 100gr. red cabbage contains
1.7g glucose, 1.4g fructose and 0.6g sucrose (sucrose), while comparatively 100g melon contain
2.6g glucose, 2.9g fructose and 2.4g sucrose.

Also, other vegetables that are sweeter after cooking are sweet onions, red sweet pepper, cabbage, carrots (they contain 3.5g of sucrose per 100g), corn, etc.

How is this phenomenon explained?
During cooking, there is a breakdown of the cellular structures (membranes) of these plant foods, resulting in the release of the above-mentioned vegetable sugars, which were not available in their raw version.
As a result, the sugars released by cooking, come into contact with our tongue, offering the sweet taste in our mouth.

Sourced from Nutrition Data: Vegetables high in fructose/sucrose:

https://nutritiondata.self.com/foods-011011000000000000000-2.html?fbclid=IwAR1rf2ujmnnvQbapre-Op38UWZ_BZEEe6xbZT9RkNQSAzqbLUQblUJluYns