We add time estimates to all of our tasks/sub-tasks during backlog grooming. A handful of our cards that have valid time estimates are showing 0h on the scrum board. Below is an example of a card showing 0hrs.
If we open the card we can see that there is a valid time estimation of 1d 1h.

I've also verified in the cards worklog that there hasn't been any time logged on this card yet that would decrease the estimate. Can anyone think of a reason it's showing 0hrs?