The reported behavior is not reproducible in the Grid's Hierarchy demo, and I am also unable to replicate it locally. Attached you can find a sample project, in which the last row of the Grid expands as expected. Could you modify it accordingly so that it demonstrates the issue and attach the example back for further review?
If you have custom CSS applied to your page, try adding it, because it might be overriding default Kendo UI styles and thus causing the unexpected behavior.
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