If the destination layer hadn't keyframe, a new keyframe was added and later the merge layer strokes were added, but this could change the animation becaus ethe new frame replaced the old drawings of the target layer.
Now, before merge the layer, all keyframes are added in the target layer in order to keep the drawings.
Original Layers:
{F9505104}
Actual result after merge layer:
{F9505107}
The correct result must be this:
{F9505110}