Created attachment 149938 [details]
Recording of the mismatch

SUMMARY
Plz, refer to https://krita-artists.org/t/when-the-angle-of-brush-tip-is-linked-with-actual-holding-angle-the-rotation-of-canvas-cuase-the-mismatch/42828 for a more explanatory description.

Using "WaterC Flat Decay Tilt" allows you to change the brush-tip angle by changing your holding angle.
Consider holding the pen so that the projection of the pen parallels the up\bottom edge of your tablet.  
When you change your holding angle by creating a 30-degree angle between pen projection and table edge,
the brush tip will rotate by 30 degrees. (30 can be replaced by any angle between 0 and 360)

If we treat the "parallel" situation as 0-degree default, then, normally, when the projection is paralleled with the tablet edge,
brush tip ('s direction, the default direction in this case) is paralleled with the Krita application window's up\bottom edge.
This should be the desired behaviour.

But, when Pixel Brush Engine - Color - Airbrush is activated, the brush tip will apply the rotation base on a default direction that is paralleled with the canvas up\bottom edge, not the window's top\bottom edge.

STEPS TO REPRODUCE
1. Select WaterC Flat Decay Tilt as your brush
2. Rotate the canvas with a noticeable degree 
3. Rotate your holding angle, and observe the mismatch.

OBSERVED RESULT

The angle of pen-holding and brush tip don't match.

EXPECTED RESULT
The angle of pen-holding and brush tip match.