So I noticed this happening on the vanilla NPC's too after testing my Mod NPC's, but I put onto them the check box option of PC Mult lvl because from what I've read that's the only way they can level up with the player. Anyway, whenever I view them after leveling it, it seems they stay at the same level that I've met them before.

Is this a bug? If so, is there a way to fix it? Or is this natural, so the player can move on to other followers?