I am seeing this as well, also on Mac, in both the Miro Desktop App and Safari (not tested on Chrome).

Steps to reproduce:

1. Drag a Card with pressed Option/Alt key. As expected, the card changes to an outline and can be dragged around with the mouse cursor.
2. Drag the card outline over a Kanban board. As expected, an empty slot will be visible in the cell the mouse is hovering over.
3. Drop the card onto that cell.

Expected result: Card is duplicated onto the Kanban board, moving into the newly created slot.

Actual result:

• The duplicated card is briefly visible but disappears immediately.
• The duplicated card exists, but is not visible until restarting the Desktop App or reloading the browser window.
• The duplicated card is immediately visible in the Desktop App if the card has been “copy dragged & dropped” in an open browser window, and vice versa.
• The duplicated card is not slotted into the Kanban board cell that it was dropped onto.