If you are shielding while going through the nether portal the game still thinks you are shielding when you get out of the nether portal even though you are not holding shield. You can still attack and shield takes damage, but you won't be harmed. This does fix upon relogging or using the shield again.
This is the link to the video since its too big to fit the attachment https://streamable.com/mf6ncm
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duplicates 1
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Duplicate of MC-86252
Tested it on the latest snapshot 1.16.3-rc1 and still works, however the shield's durability goes down extremely fast in both versions.