When sprinting, if you press crouch and don't stop moving between these actions, you're able to move faster than you should be while crouched. This also holds up when crouch jumping, making a sprinting crouch possible. If you stop moving during this effect, your normal crouch speed comes back and the bug stops. If needed I can get a recording to show what I mean at a later date.
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Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-152728, which is a report that has not been resolved yet.
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