Currently Krita's assistants work this way:

• user starts painting
• if the assistants are present and snapping is enabled, it still paints in the first pixel (or not at all?), because it doesn't know how to snap the current position to the assistant, because it doesn't know how to snap yet
• after around 4 pixels, it finally decides on which assistant/direction to use
• the delay results in a straight line at the beginning of the stroke (https://bugs.kde.org/show_bug.cgi?id=436422, https://bugs.kde.org/show_bug.cgi?id=440108)

The solution would be probably to somehow delay painting until the direction/assistant has been selected, but then paint it as if it knew from the very beginning which one it is (recreate the correct stroke).