In that last screenshot, as you fill that in, do you manually associate it with Vendor A or attempting automatically?
I went back to the other thread that you already posted a screenshot of this table. There is a Vendors column marked Ref. How does that get filled in? Because Vendor A was the initial selection on the first form and carried to form 2, form 3 won’t get it automatically. So if the ref back to Vendors never got filled in, that would be why it never gets populated in the drop down and “disappears”. If you were to finish saving the form all the way through, and then check your data, you would see that the new item was there but not linked to a vendor. I am 99% sure about this.