It will be hard to determine the reasons behind the issues you are experiencing from the provided information and screenshot.
Please refer to the attached sample page with similar to yours scenario with select functionality and "ExpandAllItems" method called on initial page load, both working as expected.
Moreover, you could have a look at the following online demos:
In order for us to be of any further assistance, please open a regular support ticket with sample, runnable project attached that replicates the issues you are observing, so we could inspect the issue locally.
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