After going through a nether portal while blocking, the shield will automatically block damage even when down. This is kind of overpowered, as you are unkillable by mobs until you block for real.
Steps to reproduce:
Enter a nether portal while blocking with a shield in the offhand.
Release right click as it's loading terrain.
Get attacked to see the blocking.
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However, we are already tracking this issue, and this report is a Duplicate of MC-86252, which is a report that has not been resolved yet.
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