J is for Jidai-geki (2012)

An executioner prepares to decapitate a samurai; however, the samurai begins performing a series of bizarre and physically impossible facial expressions, causing the executioner to panic. A man off-screen tells the executioner to finish the job, as he notices that the samurai is performing seppuku. The executioner, who turns out to be a kaishakunin, beheads the samurai, but then laughs at the ridiculous expression that the head made after it landed on the floor.

Unfortunately the movie J is for Jidai-geki is not yet available on HBO Max.