It looks to me like the insulation boards haven`t been primed. A weak mix of SLC won`t help either. I wouldn`t get involved unless they are willing to have the ufh taken up, floor primed, cable replaced and re screeded.
Looking at the discolouration on top of the screed, it looks as though waters been added, but I suspect not enough. I reckon the builders treated the SLC as a cement screed, and only added a minimal amount of water. As previous comments have suggested, carefully lift the electric mat, dust it off, relay, and then rescreed