I've never really used illdrain much before, but a desire to start influencing and not be helpless against telepaths prompted experimentation. The first time I used it, it worked like I thought it would, I'd illdrain a corpse, and depending on how strong that corpse was decided how much ego was absorbed. Then, I logged off. When I logged back on, some 8-10hrs later, I still had a remnant of the boost, but it could not be bolstered any further. In other words, no matter how many corpses I absorbed, nothing changed.

Is this a bug, or do I just have no idea how illdrain is supposed to work?

Its a bug.