Per WAI-ARIA 1.2, the "aria-level" attribute, if present, "is an integer greater than or equal to 1".
However, Blazor TreeView uses 0-indexing for the attribute which means that the root item has an invalid (and ignored) aria-level property. This is problematic because the browser will infer the level from the DOM nesting for the root item, but then use aria-level for all other sub-trees, leading to inconsistent levels (in the demo, the level goes 2 > 1 > 2).