Problem with onkeypress event

I have problem with following code - trying to change Value to static value "test" on each key press. It works for the first time, but with more keys, event is triggered, but value is unchanged. Textbox has to loose focus and i have to click sideway and back to textbox and key press to get it working again. Not sure where is the problem.

Same behaviour happends when i change event to onkeypress, onkeydown, onkeyup....

@using JetBrains.Annotations

<RadzenTextBox @bind-Value="@ZipCode"
               Style="width: 100%;" @oninput=@(args => OnChange(args?.Value.ToString()))/>

@code {
    public string ZipCode { get; set; }

    void OnChange([CanBeNull] string value)
        ZipCode = "test";

Works normally on our demo: