Scenario: existing board with some cards; open card details, assign a tag; tag appears as selected on both the card details view and on the card itself.
Create a new card on that board, open card details, assign a tag; tag shows as selected in details view but does not appear on the card; move away from the details view to another card and change back, tag is not assigned.
Same experience with assigning users to new cards. Comments and changes to description save as expected.
Reloading the board in more or less anyway, e.g. navigating to a new board and back, or using the show/hide archived cards button returns expected function.