Hardware issues are unfortunately common for Pixel phones. And this time, the latest problem is with the Pixel 4 XL. Reports on Reddit and Google's support forums have customers complaining about the phone's back panel coming loose. It typically starts in one corner and worsens from there. It seems to start from the left side corners for many users, but that could be coincidental.
The company seems to be shipping out replacements for those affected, but this doesn't solve it. Refurbished phones can still carry this issue. A Reddit comment from last month of a user claiming to be the general manager of a uBreakiFix store says the peeling is usually caused by a fault connector that causes the phone's battery to expand. It's a problem Google has faced before with the Nexus 6P. It's not a good look for Google as its hardware continues to face issues. While the company has excellent software, it has some work to do hardware-wise.
Source: Android Police