Yooo, i loaded your save file, and what you say is true, the bug doesn't occur anymore.
As for me, I made him a full chanter, and also i have a bunch of saved phrases. I have several chanting phrases set as A,B,C,B,A,A,B,B,C,D,D
The saved phrases just stays there, it doesnt go away even after retraining Tekehu's Level back to 1.
https://www.dropbox.com/s/4hn06kl2yebmqhr/Anderson%20%28FloodedCave%29%20%2889b377b0-9153-4e14-a47b-ec33122d19bf%29%20%28780494779%29.savegame?dl=0
Here is my savefile if you would like to help,
i'll be trying to fix it as well, ill just fiddle around see what works, (what did you mean by killing his starter chant exactly?)