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We regret to let you know that we are unable to reproduce the reported behavior in our end. We have prepared a sample for your reference and it is attached below. Please check whether we have followed the same steps to reproduce the reported behavior, If not please share a sample or a reproducing steps. That will be helpful for us to analyze and provide you a prompt solution as earlier as possible.
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In MDI form when a form is shown by calling the form.show() method the form's load event will get called only once and if it is closed and reopened it is needed to reinitialize the object of the form that is used for showing. On further analysis with the system's default form the behavior is confirmed as the intended behavior which is same as in Office2007Form. Please reinitialize the object to achieve your requirement as shown in the below attached sample and let us know if it helps you.
Please let us know if you have any concern,