If player 1 is shielding and player 2 hits them the shield appears to go down on player 2's screen but remains up on player 1's screen and acts like it is up this only applies to players hits and not mob hits.
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duplicates 1
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Modded servers/clients are not supported, FYI.
This issue duplicates MC-156600.
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The shield appears to go down on player 1's screen as well, but still blocks. This happened on a singleplayer unmodded 1.14.4 world