Is Light's Grace bugged?

I noticed something odd about my Holy Light cast time so I decided to do some testing. As soon as I get down to around 1.5s cast time on Holy Light, Light's Grace would just not refresh properly sometimes when hard casting Holy Light, and my next two casts are 1.9s before the passive kicks in again. Am I missing something, or are holy paladins broken?

EDIT: I grabbed another holy paladin and we were both able to easily reproduce this issue, looks like a really big bug. Created a a thread in the bugtracker: https://www.warmane.com/bugtracker/report/46409