The entire point of a ghillie suit is to try and blend into the natural environment and stay hidden. In Call of Duty: Warzone's case, though, it's perhaps a little too effective.
Indeed, the purchasable skin is actually making players entirely invisible, it seems. The skin appears to be glitched, for whatever reason, with players increasingly becoming harder and harder to see as the player moves further away, even when looking through a scope at a certain distance.
Eventually, only the players head and weapon will be visible, making them almost impossible to see, especially at a distance.
As for when this bug was introduced, it's hard to say, but it was released back in August of last year, so it may have been present for quite a while and just gone unnoticed. Warzone is quite notorious for its toxic cheating problems that ruin the game for many players, and this isn't the first glitch to be exploited in the game.
Recently, a bug in the game allowed players to use attacker helicopters to turn themselves invisible, so this isn't even the first time this has happened. Hopefully, this latest glitch is patched soon.