so, i was playing on my rather modified survival world (just modified with cmd blocks, no mods), when suddenly my shield didn't work, even if i was perfectly aligned.
what i tough it happened: the shield blocked my damage and i would be fine.
what it ACTUALLY appened: the creeper completely ignored the shield and got me down about 3 hearts.
how to re-create the bug: try in survival, try to block some creeper attacks with a shield. sometimes doesn't work
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Does MC-100949 describe your issue?

yes, thanks Asteraoth!
In that case, this ticket is a duplicate of MC-100949
Please clarify what you mean by "rather modified survival world". If you weren't experiencing this issue on vanilla minecraft, this report is invalid.