Primary field not showing in joined table

Hi all,

Hope you can help me out with the following. I have 2 joined tables, where the main table has the ID_Name field as primary key. See list below with all the data in the table:

When I start populating the joined table, I am able to select the drop down values from the main table. See below example:

The issue is, that when I jump to the next field in to add data, the selected primary key data does not seem to stick as off the 3rd record I try to add. The first 2 records show perfect, from the 3rd onwards nothing seems to stick. See sample below.

Have you any idea what I am doing wrong or what could avoid any data after the second record to not populate? Note: I have a paid subscription, so should have no limited in the database.

Thanks for your help