As for the report previewed in from a designer's Preview mode, you can wrap the tooltips by adding a new line inside the Tooltip.Text expression dialog. However, when it comes to displaying the report in a viewer, tooltips come without wrapping. For example, in HTML5-based Report Viewers. In this case, you can use the viewerToolTipOpening(e, args) event to create the logic behind the text wrapping.
For additional information, you might check Styling Tooltips in HTML5-based Report Viewers KB article.
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