When I enter an original estimate of less than 4h (up to 3h 59m) it is displayed (and summed up correctly in sprints) on the backlog.
However, if I enter anything 4h and above and save, an additional 4h is immediately added to it!! So 4h shows up 1d, and 5h shows up 1d 1h! These incorrect numbers are used to show the total estimates in sprints/backlog as well. So for example two 4h stories sum up to 2d (1d + 1d) in a future sprint on backlog 
This is very likely (partially) a UX-only issue, because when logging work, the correct original estimate is used. Here's an example:
- I enter 4h as original estimate, it shows up 1d.
- I log 1h of work, and remaining work is 3h (and not 7h).
Is anyone else facing the same issue? Hopefully this is a temporary defect that Jira will fix!