When you change the parameter in a nanoflow to a different entity and the parameter was used in validation feedback, the Variable field is automatically changed to the new entity. The Member field becomes empty (which makes sense). However, there is no hint to the developer that the member field is now empty and if you trigger the validation feedback, the user gets a user unfriendly error that the member is unknown.
I would prefer Studio Pro checking if the Validation Feedback Member is filled or not and giving the developer an error in the error pane for it to prevent releasing it.
If you edit the validation feedback it is impossible to save it with an empty Member, so I assume normally this was never relevant.
I have raised a ticket for this with Mendix, but they told me it should be feature request on the Idea page.