Only thing I can think of is you have some sort of caching issue. Try to clean rebuild the project, if that doesn’t work delete your intermediates folder and regenerate project files.
UCLASS(Blueprintable, BlueprintType, ClassGroup = (Custom), meta = (BlueprintSpawnableComponent))
class MYPROJECT_API UMySplineMeshComponent : public USplineMeshComponent
{
GENERATED_BODY()
};
is what I used in a blank project on 4.22 and had it show up in my custom components list.