﴿ لِيَكْفُرُوا۟ بِمَآ ءَاتَيْنَٰهُمْ ۚ فَتَمَتَّعُوا۟ فَسَوْفَ تَعْلَمُونَ ﴾

“[as if] to prove their ingratitude for all that We have granted them! Enjoy, then, your [brief] life: but in time you will come to know [the truth]!”

Let them be ungrateful for what We have given them this is an imperative meant as a threat. ‘So take your enjoyment. Soon you will know’ the consequences of your enjoyment there is a shift away from the third person address to the second.