I wasn't able to reproduce this issue and the solution attached in the thread you sent doesn't contain Flanders family so can you send us the sample you used to reproduce the issue. Also can you tell us which Telerik version do you use to reproduce it?
However, the thread you're looking into is a bit outdated so if you want to implement a 'tri-state' logic using an MVVM approach, it's best to take a look at this code library thread
demonstrating a sample solution.
All the best,
the Telerik team
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