I started 4 bean plants for the purpose of testing horse compost for herbicide. They started well in my standard Pro Mix in 4 inch containers. I potted up to the pictured containers and the addition of 100% compost. They started on a steady decline a few days to a week later. It is possible that I hardened them off too quickly. The moisture was maintained fairly dry on the top 2 inches but moist lower in the 100% compost area.

Does this appear to be herbicide damage or farmer error?

I have started a fifth seed in a mix of 50% compost and 50% Pro Mix but no germination yet on day 4.

As an aside, perhaps someone can inform me why my picture posts upside down, seemingly regardless of the orientation of the picture that I submit..

I'm leaning overwatering and/or root rot. Not sure there's enough drainage after adding the compost even with the promix from starting. I'd consider pulling the worst one and see what the root situation is.

Good news is your picture opened to the correct orientation when you click on it.

It doesn't look like herbicide to me.
Dump the pot and give it a look.

Herbicide damage would result in more curling of the stems. Operator error is my guess.

Was the horse compost aged? It almost looks like nitrogen burn to me.