I'm not overall sure of the cause, I think it happened a few times in the overworld but it was very noticeable in the Bastion Remnants in the Nether. I'm not sure which factors are and aren't related but here's what I did:

I was in 20w16a, creative mode, with autojump on, in the cramped bastion remnants. Sometimes a jump into a cramped block would trigger my view to sink down as if I was sunk a block deep. This happened several times until I checked F5 and found the swimming animation to be active as this was happening.