I’m pretty sure this is because you have nanite disabled in game.
The bush in UEFN is the version of the mesh with nanite enabled, which is why it looks different.
The same goes for most foliage in Fortnite. So make sure to have nanite turned on in your graphics settings. FYI: both variants are used, high end settings with nanite see the detailed version, where as the lower end settings see the less-complex ch2 version.
Trees are one of the few ways to check it out in editor.
Place down any tree that supports nanite (mainly from Ch4 onwards), and turn on the setting “Allow Masked Materials” in the world settings under Nanite > Nanite settings.
turning on/off this setting enables masked nanite materials for the world. You’ll also see all the trees change to their more detailed nanite versions. (check images)
I do want to remind you of the fact that some trees don’t change colors when tweaking the color settings in the trees. Some only change it for their nanite versions, some only for their non-nanite versions. So i’d recommend to use that masked material setting to check if both variants change their color correctly.