Has anyone ever seen this before?

I have never seen this before its like it just quit growing after node 2? There are no signs of any growth coming from the top of this lemon diesel seedling, after its first node that was regular the second split but only grew single finger leaves in place of anymore actual stem growth... Also this seedling only produced those single finger leaves unlike its other seedling siblings which are producing 3 finger leaves from the beginning

Give it some growth time then pinch that leaf off, you have two side shoots starting.

I would put an oscillating fan on them, to strengthen the stems, they are going to fall over from the upper weight.

Patience mate, on the second picture there are clear signs of nodes/branches, so I would not worry about the top being like that, some plants just grow out slow, weird, mutant etc, can still give you good clones and buds...

Usually the meristem is at the top of a plant. as others mentioned yours might be lower. It is odd, but meristem anomalies happen. I've seen seedlings "dead end" for good, they grow a set or two of leaves and then it's like they run out of meristem and go brain dead. Some other weird shit that happens is a Siamese twin seedling or buds growing from the base of a leaf almost from the petiole.

As an aside, you might want to check your seedling for mites, looks a little speckled. Good luck.

Very good answer. I had a plant or two not be able to grow past a dead end point. Which was very strange but not the type of genetics I want to keep in the garden anyways. Yours looks like it is sprouting two branches one node back. You probably don't have to do anything. Looks like it is already redirecting it's growth.

I think you have a triploid mutant. Never know what's going to happen. I would keep tending it. Maybe you have the all-time killer plant.