What essentially had happened was the Rug was holding the heat in a confined space which resulted in the LVT scorching black under the rug area only. Obviously through the temperature exceeding the LVT specification in this limited area.
The reason for it over heating of course is the bit that’s debateable and when discussing with the colleague, I speculated that the UFH temperature sensor in the floor and the air temp sensor were not calibrated to each other or one or both wasn’t working or was abscent from the system for whatever reason.
Either way, the floor was over heating across its entirety to try and reach the temperature set on the thermostat sensor and/or the rug specification exceeded the parameters of the LVT over UFH.
Far less likely to have discolouration issues with stone or ceramic.
Replacement/repair of the floor was proposed but as is so often the case, the builder responsible for the UFH installation was struggling to provide the correct specs for the installation and where the sensors were etc etc etc.
So basically everyone was blaming each other and no one wanted to commit to replcement/repair unless they could guearantee it wouldn’t happen again.
When using rugs over any flooring, there’s always the risk of fading and discolouration of some description showing when the rug is lifted LVT is far more susceptible being a plastic/vinyl than ceramic.
This is an interesting thread on simple sunlight damage that can always happen but probably should have been discussed or researched more by the retailer/consumer
https://theflooringforum.com/threads/harvey-maria-lvt-how-was-this-stained.17605/