Your tempering is fine. Your poured couverture is not cooling down fast enough and is suffering from "latent heat build up", which is why the edges are in perfect temper, but the middle is out of temper.
Is the poured slab resting on a hot counter or a hot room?
Can you try to put the poured slab in the fridge for no more than five minutes?
Is the poured slab resting on a hot counter or a hot room?
Can you try to put the poured slab in the fridge for no more than five minutes?