It’s less like a U and more like a wave whose amplitude gets less and less every time until you stabilize in your new country (the peaks and valleys get smaller and smaller). It does also repeat if you go back to your home country (reverse culture shock), but the frequency is less and the total stabilization time is lower because for the most part, your brain is relearning cultural cues the connections already exist for, with an allowance for cultural drift depending on how long you’ve been away.