When you have the RadSplitter configured in percentages, all its parent elements including the body, form and html should have their size explicitly set either in percentages or pixels in order to let the splitter calculate the size it should take. This is not new and I do not know why it worked in your previous version - you can see a sample demo explaining this below:
This being said, you will get the desired behavior not only by adding the following style to your page:
html, body, form
as explained in the demo but also set all teh parent's height, e.g as shown below:
<div style="height: 100%">
On a side note, I also suggest to set VisibleDuringInit=false for the splitter - this will hide the initial resizing of the splitter from the end user and you will get a better behavior.
I hope that my explanations and suggestions are helpful, let us know if you need further assistance.
the Telerik team