RadzenTabItem has different design in new Radzen Components 9.01
when I revert to Radzen 8.75 design is as I expected
The rendering or various components is changed in latest version to improve WAI-ARIA support and semantic HTML. Here is the full list of changes: https://blazor.radzen.com/changelog
To fix these you'd have to rebuild your theme from the Appearance panel - simply change to any other theme and then back to the one you were using.