Hey--this technically isn't a bug, but I think it could use some fixin'. The main purpose of the Humbaba is to provide a buff while you're crushing stuff...except that, if your mood is 100%, you can't drink one. I was on Rookswort with 100% mood, thought I drank a Humbaba, didn't realise it hadn't been drunk, then crushed a bunch of rock with no buff. :/ I had to wait to continue crushing until the Rookswort buff wore off and my mood dropped.
I'd like to suggest adjusting this so that you can drink a Humbaba regardless of your mood level.