Modification of a public property - LWC

As per documentation:

We recommend using primitive data types for properties instead of using object data types. Slice complex data structures in a higher-level component and pass the primitive values to the component descendants.

There are a few reasons that it’s better to use primitive values.

Primitive values require specific @api properties that clearly define the data shape. Accepting an object or an array requires documentation to specify the shape. If an object shape changes, consumers break.

Standard HTML elements accept only primitive values for attributes. When a standard HTML element needs a complex shape, it uses child components. For example, a table element uses tr and td elements. Only primitive types can be defined in HTML. For example, isn't a value in HTML, only in Lightning Web Components.

This is definitely a bug because according to documentation,

When you add a component in markup, you can initialize public property values in the component based on property values of the owner component. The data binding for property values is one-way. If the property value changes in the owner component, the updated value propagates to the child component.

The child component must treat property values passed from the owner component as read-only. If the child component tries to change a value passed from an owner component, you see an error in the browser console.

To trigger a mutation for the property value provided by the owner component, the child component can send an event to the parent. If the parent owns the data, the parent can change the property value, which propagates down to the child component via the one-way data binding.

Also in the same documentation, you will find example of todoapp and child comp todoitem, in which its clearly written that

Nothing happens in the UI! You see an error in the browser console saying that you can’t change the value of itemName. You can’t change itemName in c-todoitem because you can only set the value of a public property (@api) at component construction time.

So, whether the properties are complex or primitive data-types, @api public properties in child should not be allowed to be modified after the construction time when they are passed from parent component.