1- Component-Specific Styles: Angular encapsulates styles in components by default (via ViewEncapsulation.Emulated), appending unique attributes to CSS selectors. These styles are injected into the when the component is rendered. If the component hasn’t fully loaded when the page renders, its styles won’t apply immediately.

2- Lazy-Loaded Modules: If your routes (or the sidebar component) are part of a lazy-loaded module, Angular loads the module and its styles only when the route is activated. This delay can cause unstyled content to appear briefly.

3- Router Outlet Timing: The dynamically inserts components, and if the routed component or sidebar loads after the initial DOM render, styles might not be present at startup.