Flying + sideway movement = weird y movement depending up/down camera direction

[blablubbabc]

Just noticed while playing around with the web demo:
When you are flying and you only press A or D (move sideways) while looking straight down it moves you downwards, instead of sideways.
The y movement (up-down) actually depends on the view direction: the more you look upwards, the more you move upwards when walking sideways.