Seam Separation Problems with Vinyl Plank Flooring
Is this a glue down installation or a floating installation? If it's floating it sound like failure of the joint, either the locking system or at that age, more likely a the glue strip it it attached with. If it's a glue down install it's probably something moving in the substrate. If it were the material itself I would expect it to happen sooner than 5 years.