There have been several errors inside the builder and the app, for example there are tables that simply disappear both in the app and in the builder, also some fields of multiselection call data that does not correspond to the field, recently create a drop down menu and it disappeared from the builder However in the app is still visible, someone else has occurred any of these errors or something similar?
I constantly run into issues like your example. For instance, it just happened to me a few minutes ago. I was creating a Text Formula and only two values show up, when I know there are others. I log out and come back in and it is fine. The same thing occurs when you create new fields, or even new pages. They typically do not show up right away, but if you refresh the screen they usually become available. If not, try logging out. I know this is annoying and frustrating sometimes, but at least it's consistent. It will be nice when Knack automatically refreshes this information while in builder.