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TreeView doesn't update nodes recursively when setting CheckedNodes initially and adding at runtime since treeview populates the nodes in on-demand. Refer notes section of below documentation,
https://help.syncfusion.com/xamarin/sftreeview/checkbox#get-or-set-checked-items-in-bound-mode
Need to add support for recursive checking of CheckedNodes for programmatic cases also when NodePopulationMode is Instant.