ToME WikiKalirion: Dropping a hypnotized pet and releasing it levels it up by one level. Recalling to a dungeon and dropping it and releasing it can level it up by multiple levels. Specifically, every time I recalled from town to Tulkas' temple level 33, and dropped/released the symbiote, it leveled up by around 15 levels. In order to keep getting the large jumps, I needed to recall back to town, and then back to the dungeon again, but still got a level 137 Trapper in no time. Otherwise if dropping/releasing in the same dungeon over and over it levels by 1 level again. Haven't tried it in other dungeons, maybe the auto-leveling of Tulkas' temple has something to do with it. When this type of leveling happens, the pet's hitpoints stay at their original level, but Probing reveals that other traits like AC, speeed, and attacks all increase.
ZizzoTheInfinite: [sound F/X: source diving] Looks like the symbiote's experience points are being double-counted on release, hence the level gain. The bug is still present in v2.3.2; I've put together a patch that appears to fix the problem.
Now, the monster-leveling effect of the temple is a separate issue (though it was likely compounding the bug and making it worse). I haven't tested the patch in that situation yet; the intended behavior is that the level of the released symbiote will be the larger of its level when it was hypnotized and the level it got leveled to when it was generated. Which means that releasing your symbiote in a monster-leveling dungeon is still a quick way to level up your symbiote quickly (mitigated somewhat in that it will gain no extra hitpoints in the process). I'm not sure what the correct behavior is here; if we want to prevent the symbiote from leveling in this case, that will probably require a new parameter to place_monster_one() in src/monster2.c to disable dungeon-based monster leveling.