Now that I’ve gone and checked. It’s just that the hand looks lower down than in should be compared to Java in first person.
To reproduce:
Join a world making sure you’re in first person. Now look at the hand.
As you can see in the images uploaded below. The one with the arm lower down is what it currently looks like, and the other is what it should look like (fixed with a resource pack).
Also, I only know that this is true for iOS, Android & Windows. I don’t have the ability to check others.
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I do apologise for describing this bug really badly thinking it had something to do with the bobbing animation in first person. In reality, it was the fact that the classic armed skin would render as the same width visually as slim-armed in first person. Like it would still be 4 pixels wide, but it would just look a bit squashed compared to how it renders on Java Edition. I'm unsure if this has been patched in the current version since I'm unable to check in the current time, apologies.

We are tracking this issue at MCPE-47877, so I will resolve and link this ticket as a duplicate.
Thank you for your report!
However, this issue has been temporarily closed as Awaiting Response
Is this still an issue in the latest version? If so, can you please add it to the affected versions (or mention it if you are not the reporter).
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