Creepers unable to swim properly/ Players unable to swim up though one high air gap whilst sneaking

Summary

Basically if you make a vertical shaft (1 by 1) and place water and signs like:

mobs that are taller than a block should be able to swim up it easly however creepers are getting stuck bobing up and down in the space with the sign in (even though there hitbox is in the water above them) see video: https://www.youtube.com/watch?v=0sqlgPK-MXA&feature=youtu.be

EDIT:

this also affect players whist sneaking ONLY which is inconstant behaviour Shown in this vid: https://www.youtube.com/watch?v=h6-Ne7Wo-wY&feature=youtu.be

EDIT 2

added an world where you can automatally test if the bug still affect that version (called 'Bug test World.zip)