If you are using AJAX (e.g., to bind the list when a tab expands), can you confirm the tooltip manager is included in the partial postback? The easiest way to test this is to disable AJAX temporarily and use full postbacks. If things work fine with full postbacks the most likely reason for the issue is that the tooltip manager needs to be updated as well.
In case full postbacks do not work as expected, does taking the list out of the accordion control and removing the AjaxControlToolkit assembly from the solution help? Their script manager is known to bring modified MS AJAX scripts and cause compatibility issues, so I can suggest using our RadPanelBar instead: http://demos.telerik.com/aspnet-ajax/panelbar/examples/overview/defaultcs.aspx
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