Prerequisites

• I have read the Contributing Guidelines.
• I agree to follow the Code of Conduct.
• I have searched for existing issues that already report this problem, without success.

Stencil Version

4.43.0

Current Behavior

When setting an optional mutable property to undefined initially, the host value is never updated.

Expected Behavior

Host value is always in sync with prop value.

System Info

Steps to Reproduce

1. Define mutable optional prop
   @Prop({ mutable: true }) first?: string;

2. Set prop to undefined initially

const myComponent = document.querySelector("my-component");
myComponent.first = undefined;

3. Update value of prop internally
   this.first = 'Some value';

4. Host value was not updated to new value
   this.host.first => still undefined

Code Reproduction URL

https://codesandbox.io/p/github/johnjenkins/stencil-starter/csb-z96sgg/draft/damp-sound

Additional Information

Seems to work in 4.37.1 and breaks starting from version 🎿 4.38.0 (2025-10-02)

Problem seems to also exist when the prop is not mutable and the value is set from outside but the other way around. The internal prop is not updated in that case.