We'd need to know what options were selected in defining your value list and the exact values you've entered that should be part of this valuel list, to be sure, but...
I suspect that your field is of type number when it should be of type text given that adding a numeric character changed the behavior of your value list.
You might have unwanted carriage returns in your list. For example, to create a value list that has the values "yes" and " " (blank) you would enter "yes" and press Return twice. Rereading your post again, it's more likely there are entries in your related table that are populated by a space. Or . . . Phil's answer!
Phil nailed it. I was selecting a calculation field (first name last name) and FM doesn't do this very well, as proved by the erratic behavior when I added a numeric at the end of a last name. I switched to last name only and all went fine.
The calculation field should work just fine as long as two requirements are met:
The field should be stored so that it's index can be used to produce the value list.
"Text" should be specified in the return type drop down in the bottom left corner of the Specify calculation dialog.
Then there should be no "erratic behavior".