I built up a test demo based on your code but unfortunately to no avail - I was not able to reproduce the problem - please see the video capture I prepared and attached for your reference.
The content did not disappear but I noticed that your splitter does not occupy 100% of the height of the page - in order to correctly size it you should add the style Chris suggested. As to the article he provided, this used to happen only with the Q3 2008 release and happens on page load when VisibleDuringInit=false which is not your case.
In case the problem persists, please provide more detailed reproduction steps, explanations, browser infirmation and some screenshots and once I am able to observe the problem locally I will do my best to help.
the Telerik team