When longer string is used for HeaderText the text could not be wrapped. Also the issue occurs when using Label with LineBreakMode="WordWrap" inside the TabViewHeaderItem.Content:

<telerikPrimitives:TabViewItem>
    <telerikPrimitives:TabViewItem.Header>
        <telerikPrimitives:TabViewHeaderItem>
            <telerikPrimitives:TabViewHeaderItem.Content>
                <Label Text="Lorem ipsum dolor sit amet, consectetur adttert etertert erterter ipiscing elit." LineBreakMode="WordWrap"/>
            </telerikPrimitives:TabViewHeaderItem.Content>
        </telerikPrimitives:TabViewHeaderItem>
    </telerikPrimitives:TabViewItem.Header>
    <telerikPrimitives:TabViewItem.Content>
        <Label Margin="10" Text="Label" />
    </telerikPrimitives:TabViewItem.Content>
</telerikPrimitives:TabViewItem>