For :hover transition, you must set 2 transition for unhover same effect, or it will work with single for parent property

... {transition: background-color 600ms ease-out 150ms;}

...:hover {transition: background-color 600ms ease-in 150ms;}

transition: [transition-property] [transition-duration] [transition-timing-function] [transition-delay]; - short variant

css tricks

when using transition, you must set, what parameter to transform, e.g. width, height, color, background-color..., it must be set for both parent and parent:hover, if several properties must be animated, [transition-property] must be "all" or empty