This is similar to the Blazor Splitter problem reported on May 5, 2022. I am using a Web Assembly application rather than the Server version you supplied in your response to that problem.
The splitter can be resized for all versions up to 19.4.0.56. It does not work for any 20 version up to 20.3.0.60
You should be able to run your 'Getting Started' example as a WebAssembly version, not a Server version to see the problem.