That is my semi educated guess.
Luxury vinyl plank not sitting flat.
Level is not necessarily important.
Even though the first reason for the invention of vinyl planks was to mimic hardwood floors today it authentically mimics different textures colors and species.
Your floor is not flat so the flooring is holding itself up by it s own will and when you walk on it you are pressing it down into those valleys and it s touching the concrete.
After all a floor can be flat and have a plane to it.
Floor prep the only major rule for installing vinyl plank floors is that the surface needs to be flat.
When not installed on a level surface a luxury vinyl plank floor can shorten in length when it rides over high and low areas and it can also lift on edges or ends where a substrate irregularity.
My new click lock floating luxury vinyl plank floor has some dips and noises i wasn t expecting.
Vinyl plank is water resistant and is not like hardwood which is not resistant to water.
Vinyl plank flooring over radiant should not be a problem provided you follow the manufacturers recommendations.
Primary issues arise in some systems with uneven surface temperatures causing the flooring the react inconsistenly to the wide changes in temperature.
During installation run a vapor barrier under the vinyl plank flooring to help prevent.
You can seal vinyl plank completely and prevents moisture from going into the floor.