In a multi-level menu, you usually have to highlight the selected menu item and its parent element. This can be done based on the parent route.
<ul>
<li>
<a href="{{ route('admin.posts.index') }}"
class="{{ Str::is('admin.posts.*', Route::currentRouteName()) ? 'is-active' : '' }}">
Manage Posts
</a>
<ul>
<li class="{{ Route::currentRouteName() === 'admin.posts.index' ? 'is-active' : '' }}">
<a href="{{ route('admin.posts.index') }}">All Posts</a>
</li>
<li class="{{ Route::currentRouteName() === 'admin.posts.create' ? 'is-active' : '' }}">
<a href="{{ route('admin.posts.create') }}">Create Post</a>
</li>
<li class="{{ Route::currentRouteName() === 'admin.posts.settings' ? 'is-active' : '' }}">
<a href="{{ route('admin.posts.settings') }}">Settings</a>
</li>
</ul>
</li>
</ul>