In January 2017 I reported this behavior as well:
Bug Behavior with Related Reference fields from a source table with a Date-field as the Key
When you have a table with a Key Field as a Date field; and build a relationship to a child table.
When you first create the child; your selections are normal (per your record picker). However; when you save the record without making a selection and then edit the record; the drop-down shows a nonsensical unknown value.
QuickBase response:
"Sorry to report, but you've run into a bug.
QBE010296: Date Time as Reference results in incorrect values in child table "
I am pretty sure your issue is the same thing.
