Its debatable whether its a bug in the first place and PDX does not seem to want to clarify whether it is one.
Because it is working as intended.Why this dont already fixed ??. LoL !!.
Citation needed.Because it is working as intended.
Every military building works the same way. They clearly do not want the HRE to get +zillion% to Heavy Infantry stats just because all their vassals built a barracks.Citation needed.