This issue is no longer reproducible in ArcGIS Pro 2.5. The hover-text appears as expected in the Bookmarks gallery control for bookmarks with long names, and with/without description text. We do not explicitly code-to-fix this equivalency problem, so it is addressed by other code install.
解决办法
Open ArcGIS Pro and zoom to any extent that is not the full extent.
Once at the desired extent, navigate to the Map tab in the ribbon.
Click Bookmarks > New Bookmark.
In the Create Bookmark window, specify a name longer than 18 characters (spaces included) and state the description.
In the ribbon re-select Bookmarks.
Hover over the newly created bookmark that has a description.
Be sure to hover over either the bookmark image or the text below it.
Observe a message showing the bookmark full name and description.