The Bug:

Players' arms follow the position of sneaking players when blocking with shields.

Judging by the assigned Mojang Priority on MC-190817, players' arms should not follow the position of sneaking players when holding loaded crossbows or fully charged bows. With this in mind, one would expect players' arms to not follow the position of sneaking players when blocking with shields.

Steps to Reproduce:

1. Obtain a shield, hold it, and begin blocking with it.
2. Switch to third person and begin sneaking, and as you do this, look at the position of the arm that's used to hold the shield.
3. Take note as to whether or not players' arms follow the position of sneaking players when blocking with shields.

Observed Behavior:

Players' arms follow the position of sneaking players.

Expected Behavior:

Players' arms would not follow the position of sneaking players.

Code Analysis:

Code analysis by Apollo30 can be found in this comment.