Hi Julián,
I was able to test using an IAP-103 with standard software release version: ArubaInstant_Pegasus_6.4.4.8-4.2.4.6_58505
The behavior on a laptop is the video will not play (website loads)
The behavior on a smartphone with the Youtube app is like you described, the videos play.
My ACL on my IAP looks as follows:
all dhcp to all
deny application youtubeeducation to all
deny application youtube to all
allow all
If I add deny streaming to all above both youtube ACLs, it still allows me to stream youtube from the APP.
I suggest you open a TAC case.