I use the primary field to display the content of another field (single select)

In the primary field, I have a formula that is the other field name (ML ID)

When I add content to ML ID  it saves and displays ok. However, it doesn't show up in the primary field. The only way I can get it to display in the primary field is to delete it in ML ID and then select it again, whereupon the primary field displays it.

I attach screenshots
14.51.48 shows the field ML ID in which I've added abc
14.52.59 I've removed the abc (by selecting the blank or empty) shows the primary field which displays (for no apparent reason) --
14.53.20 shows the primary field and ML ID field as it should be


Any suggestions, please?