minecraft bedrock doesn't allow you to crawl like you can inside of java.  to mine with a single block height you need to start swimming inside a block of water then you can remain in the swimming mode to mine single block.

however if you save while inside this state, when you are returned to the surface height rather than the location you were at.

for example, you could be a location 10,10,10 when you save the game, but when you reload, you will be at 10,65,10.  so you are now on the surface of the map rather than down at the 10 Y co-ordinate.

this is compoundly erroonious when working with a minecraft bedrock server.  as the service can sporatically disconnect you, only to reconnect at a different location to where you were.