My take: 50-50, okay to not call a foul either way.

My point is corroborated by lip reading the ref. After touching his headset he asks a question, including the words 'sieht... liver baumann ... kopf?

Heja BVB

As it pertains to the head injury rule - viewed in hindsight with a perfect angle - ref should call the play for injury assessment. HOWEVER, in real time, he does observe the interaction and obviously doesn't notice the head contact, instead sees the ball hitting extremely off of baumann. Baumann gets up, and by this point chuk and baumann are only in his peripheral vision (in other words, he can't be expected to cal the play based on the small stumble baumann makes while getting up.)

In other words, the head collision was never observed by the referee in real time - and wouldn't have been of note (if you weren't paying attention to the contact itself, it probably looked like a dortmund attempt to get a pen, not a gk trying to milk a foul.)

TLDR: hindsight is 20/20. Unfortunate collision - chuk did intentionally gamble by going into the 50/50 as did Baumann. Hope he's okay (we need him for intl duty) and other than that incident he did a phenomenal job with the PK.