My apologies. As a relative newcomer to Dorico (who has not needed to use chord symbols or diagrams yet), I was not aware that it was possible to place chord diagrams at the start of a flow, before the actual music. I have just tried what you described (now that I know what to do) and have replicated your experience. I’m sure Daniel Spreadbury will be investigating.
When I selected the Dm7 chord diagram above the staff and used alt/opt-Q to cycle through the various possibilities, the diagram at the start is indeed missing the barré indication.