I encountered the same issue earlier what i think happens is the following. Between the time that the microflow is triggered and the form is loaded mendix does not detect the usage of the np entities and thus decides to clean them. What you can do to resolve this issue is create an reference set association to your master entity. That way the owner of the assoc will change and thus mendix will not clear your master object. This can only be done if hour master object is also np because mendix does not allow you to draw an assoc from an persistable entity to an np entity.
Is this always when going to a specific page, or does this happen only sometimes.
does this question help you: Question on MxForum