Steps to Reproduce:
1.Join World
2.look third person arm bob animation

Observed Results:
No up-and-down swaying, only side-to-side swaying

Expected Results:
Rocking up and down just like Java Edition.

I wrote these as players, but they also affect humanoid mobs.

The number here... in "query.life_time*(here...)" should be 103.13244 instead of 103.2.

So the code looks like this:

"animation.player.bob" : {
  "loop" : true,
  "bones" : {
    "leftarm" : {
      "rotation" : [ 0.0, 0.0, "-((math.cos(query.life_time * 103.13244) * 2.865) + 2.865)" ]
    },
    "rightarm" : {
      "rotation" : [ 0.0, 0.0, "(math.cos(query.life_time * 103.13244) * 2.865) + 2.865" ]
    }
  }
}

Also, the X Rotation animation has disappeared.

This issue has been around since 1.2 and Bob on the leftarm is no longer present as of 0.14.0.

This is an issue that hasn't been fixed, even though player animations are now accessible in resource packs in 1.13.0.

It is set to "Plausible", but you should be able to see that the x rotation is gone from the bob animation, compare it to the Java Edition.

It appears that the removal of the leftleg animation was an intentional change.