I have tested the online demo that you are referring to, but I was unable to replicate the issue that you have described.
Could you please ensure that you are not clicking on the next month days, which are with lighter color in the currently selected month view. Those lighter dates are from the next or from the previous month.
If the above is not the case, could you please record a video demonstrating the issue and provide some details on your local settings (culture, etc.).
I am looking forward to your reply.
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