The Grid will cancel the insertion of a new item when:

• the new item is not saved yet
• the new item is expanded and its DetailTemplate is visible
• the user changes the value of a column that has an editor template

Blazor REPL test page

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In the meantime, a possible workaround is to hide the expand button for items, which are not added to the "permanent" Grid data yet. Here is how to do it with CSS:

.k-grid-add-row .k-hierarchy-cell .k-icon {
    display: none;
}