If you have your shield up while entering the nether portal, you can sprint and attack while your shield is up.
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duplicates 1
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Since this can be reproduced in a vanilla environment, this report appears to be a duplicate of MC-86252, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.
I did not find this. Reddit user u/OmNomZom found it.