In version 1.12* , if a hostile mob's head was inside a transparent block such as glass, it would not attack players. This is no longer the case in 17w49b.
MC-122988
Hostile Mobs with head in transparent block attack players
Resolved
Works As Intended
1
Xavom
Unconfirmed
Minecraft 17w49b
Retrieved
Does not work as intended anymore in 1.13-pre7. Mobs no longer attack players when their heads are in transparent blocks.
Probably because of bug fix MC-124126