If the leak is being caused by a problem with the gasket or seals under the flush valve, the leak might stop as the seals gradually relax and form themselves to their final shape.
There could be a small crack or flaw in the plastic of the flush valve.
Try a different flapper that fits your flush valve.
Maybe a squishier one will seal better.
Try feeling the entire rim of the flush valve for any defects, it can be sanded lightly with very fine sandpaper.
Also, the link that attaches the flapper chain to the flush lever looks to be installed wrong.
That link looks hung up.
The squarish fatter end should be hanging off the hole in the flush lever.
So that big boxy end can move freely as the flush lever goes up and down.
The end with the opening should be hanging down, with the chain hanging from it.
Also, sometimes you have to move that link to a different hole on the flush lever, to get the right angle of pull to make the flapper stay properly centered when it drops back down after a flush.