To investigate the issue of seeing unexpected UI behavior in one of the custom Lightning web components (LWCs) their team has built, the developer should enable Debug Mode for a storefront user, log in to the storefront, and use Browser Inspection tools and debugger points. Debug Mode is a feature that allows developers to debug and troubleshoot custom LWCs in the storefront by disabling performance optimizations and enabling source maps. Source maps are files that map the minified or obfuscated code to the original source code, making it easier to read and debug. To enable Debug Mode for a storefront user, the developer can go to Setup, enter Users in the Quick Find box, select Users, click Edit next to the user name, and select Debug Mode. After enabling Debug Mode, the developer can log in to the storefront as the user and use Browser Inspection tools and debugger points to inspect and debug the custom LWC. Browser Inspection tools are tools that are built into web browsers that allow developers to examine and modify the HTML, CSS, JavaScript, and other aspects of a web page. Debugger points are statements that are added to the JavaScript code of a LWC that pause the execution of the code at a certain point and allow the developer to inspect the values of variables, expressions, and other elements. Enable Debug Mode for a storefront user, load the LWC in Visual Studio (VS) Code, attach to session, and view debug logs in VS Code is not a valid way to investigate the issue of seeing unexpected UI behavior in one of the custom LWCs their team has built, as it is not possible to attach to a session or view debug logs for LWCs in VS Code. Enable debug logs for a storefront user, log in to storefront and perform action, and view debug logs in Setup is not a valid way either, as debug logs do not capture information about LWCs or UI behavior. Debug logs are records of database operations, system processes, and errors that occur when executing a transaction or running unit tests. Identify the user, inputs, and failure, then ask Salesforce support to investigate the issue with the custom LWC is not a valid way either, as it is not a recommended or efficient way of debugging or troubleshooting custom LWCs. Salesforce support may not be able to provide assistance or guidance for custom LWCs that are developed by third-party developers. Salesforce B2B Commerce Developer Guide: Debug Lightning Web Components,Lightning Web Components Developer Guide: Debug Your Code,Salesforce Help: Debug Logs