Based on the provided information, I am not sure what might be the cause of this problem.
That said, could you please elaborate. For example, share code snippets, a runnable project, etc.
Additionally, I would welcome if you prepare and share a runnable sample Dojo
that clearly replicates and isolates the problem.
In the meantime, you might be interested in checking the "Change DatePicker Value from Another DatePicker" jQuery article: