I can confirm, it is still present in UE 5.3.2. To reproduce the behavior, create a simple collection in Blender with two cubes. Link this collection in another .blend file and export .fbx from here. On importing in UE there are two separated cubes, as expected. After reimporting one of the cubes, the meshes get combined.
This has been happening to me since I started using unreal with 4.18 and is still present in 5.5 and I can only assume its not a bug but expected behavior? I can work around it by exporting every mesh separately… but thats a horrible way to work especially with anything that is more than 1 mesh basically.
This is the only game engine Ive used where this is a problem- is there a way to fix this? I dont have combine meshes enabled.