You cannot begin swimming when standing on non-full blocks underwater that can't be waterlogged

when walking on mud under water, pressing the swimming key does not make you swim.

What I expected to happen was...:

I expected to swim

What actually happened was...:

I started running under water

Steps to Reproduce:
1. walk under water, at least two blocks of water on mud
2. press swim key
3. you start running

pressing swim key again starts the swim, (which happens after a single press on other blocks)