With 4.1.6 the component RadzenSidebarToggle now looks like a button:
You have a theme discrepancy - using an old (pre 4.x) theme with 4.x.
I had the same but I fixed it simply by changing the theme to another then changing it back.
Even Radzen 2.80.3 seems to produce projects with this kind of discrepancy.
We can't reproduce such an issue I am afraid. If you use a custom theme make sure you upgrade it by going to the settings and clicking the save button.
The kestrel server is showing this behaviour not IIS express.
WebAssembly: The index.html produced by Radzen.exe is missing the following line:
<link rel="stylesheet" href="_content/Radzen.Blazor/css/standard-base.css">
Radzen applications include the theme CSS file via styles-generated.css.