I had realized that, in my survival world, when I was fighting skeletons, my shield would show as not blocking even though it was. Later I went into a superflat test world to confirm or deny my understanding.
I found this:
The shield, if it takes durability damage, will look as if its not blocking.
When this happens, it still blocks any damage it would've blocked. (Thus why I see it as a visual bug)
Feel free to ask further questions! You may find more info in the video attached!
This issue duplicates MC-156600.
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