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What do the function signatures inside your interfaces look like, and what does WidgetAdmin inherit? I wouldn't expect any problems because you're using the recurring generic pattern, which I have a feeling you might not be implementing correctly. You also shouldn't be calling base(page) because there's no base class to call the constructor of; do you have a WidgetBase class implementing IWidgetBase<T> that you're inheriting from in both WidgetPage and WidgetAdmin that isn't shown in the example you gave?

I tried re-creating what you've described and this seems to work fine:

public interface IWidgetBase<TSelf> : ISearch<TSelf>, IStuff<TSelf> where TSelf : class
{
    public abstract Task<TSelf> SearchAsync();

    public abstract Task<TSelf> StuffAsync();
}

public interface IWidgetAdmin<TSelf> : IWidgetBase<TSelf>, IDeleteWidget where TSelf : class { }

public interface IPage { }
public interface ISearch<T> { }
public interface IStuff<T> { }
public interface IDeleteWidget { }

public class WidgetPage : IWidgetBase<WidgetPage>
{
    public WidgetPage(IPage page) { }

    public async Task<WidgetPage> SearchAsync() => this;

    public async Task<WidgetPage> StuffAsync() => this;
}

public class WidgetAdmin : IWidgetAdmin<WidgetAdmin>
{
    public WidgetAdmin(IPage page) { }
    public async Task<WidgetAdmin> SearchAsync() => this;

    public async Task<WidgetAdmin> StuffAsync() => this;
}

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