When I was swimming under water using sprint and the jump key at the same time I swam up under oak plank slabs, they were one slab high above the water and then no-clipped through them and was able to stick my head through and move around with-out collision until I moved out of them.
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Thank you for your report!
We're actually already tracking this issue at MCPE-72648, so I will resolve and link this ticket as a duplicate.
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