Really happy to hear this is one of your favorites, that's a huge compliment!
About that buff, I'm not sure about making that buff stackable since it could get out of hand. EDIT: all buffs now stack.
For the cleansing issue, if you quit right after using the material the game may not save the stats. It could also have removed a different stat since it’s random or returned the material to your inventory if it had no effect. If none of these apply sadly I'm unsure, but I suggest trying again and if it repeats let me know and I'll look into it.
I appreciate all the feedback and that you enjoy the game a lot!