It appears that there is an issue with rendering and using the SfSplitter control in the IE11 browser. Unfortunately, that is a browser we are required to support.
First of all the slider resisze handles appear offset to the right of the split bar. Secondly, once you click to drag the slider and release the mouse button, the slider still continues to move with the mouse button and there doesn't appear to be a way to "drop" the slider handle.
I have verified this with a fresh Blazor application template and added part of one of your demo splitters to the index page. A screenshot of the output in IE11 is attached.