UI Bug after error message

I’m writing a page extension to our Project Requisition page (custom page) so that if a user tries to submit the form for a Non-Inventory item without filling out the “Deliver To” field it throws an error. The problem is that after the error is displayed it clears out the first line of the subform. If I close out of the page and re-open it then the line reappears. Has anyone noticed this happening on other pages? It seems like a bug.