Opening a form or microflow from a menu always clears the current form displayed In Content, even if the new object is opened as a pop-up.
The only workaround that may work is to attach a microflow to your menu, and as part of the action, re-open the current form In Content, as well as opening the new form as a pop-up. This may involve creating and maintaining a temporary record to identify which form is currently open In Content. If the form In Content is a data grid, you would also lose any current search selections. Not very elegant I'm afraid.