(EU-PC)
When I was tanking Molag Kena last night, I noticed that the cone attack was often knocking me back despite me holding block. I wasn't sure what caused it, but then I found people on the forums having the same issue. Apparently some spells disable block until their animation is complete (for example Ingenious Shield), despite being instant casts.
Since I haven't found a bug report about it, I thought I'll make one. Here is the original thread:
https://forums.elderscrollsonline.com/en/discussion/400246/block-dropping