Thanks for the help!
This is strange and unintuitive. Blender can’t expect me to know that these sockets change somewhat unpredictably with no indicator.
I thought the point of the dotted diamond was to show that the socket could be either be a field or a single value. I had assumed that since the ID value was hidden, it should implicitly default to a common index. At least an enum to change between single and unique index would be appriciated.