It's an intentional effort by the author to separate Purchinov the character from the real-world Putin - or more precisely, to turn Purchinov into the anti-Putin.
In chapter 43, when Purchinov is offered kingship by the griffons, he refuses, saying this:
Chapter 43, of course, was released in March 2022. The cross-shaped scar came in chapter 47.
Purchinov wasn't at any point meant to be Putin, but a satire of Putin (specifically, pro-Putin propaganda), but the authors decided to remove even the last shred of ambiguity about where their politics lie by putting a big X for "No" across his face.
Which is based out of space, if you ask me. That decision probably saved the manga, and if anything, made it even better.