To this day, we still don't know why Finn's lobby pose was changed from a unique one, to a basic lobby pose, which is the same lobby pose as the Sith Trooper. :
Fortnite x Star Wars: The Rise of Skywalker bundle with Rey, Finn & Sith Trooper skins in store - Millenium
Star Wars' Rey and Finn Rise of Skywalker skins now available in Fortnite - CNET
Fortnite Introduces Rey, Finn and Sith Trooper Skins to the Battle Royale Game - TechEBlog
Fortnite adds three new Star Wars: Rise of Skywalker skins - Dexerto