How to reproduce it:
1. Find a lake with water or an ocean
2. Start swimming and then sprint
3. Switch to Third-Person and watch tha players' arms animation
(The reason you're unable to see the Beta text on top of my attachments, is because I've installed a resource pack that removes it.)
Thank you for your report!
We're actually already tracking this issue at MCPE-66475, so I will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
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