Closing as not a bug: 'pixel rounding' has no effect on this, since it does not affect the actual object co-ordinates, only the position when rendering the object image.
See 'Expecting math calculations to be exact' in the blog post common mis-used events and gotchas. This is a normal floating-point precision issue. If you add and subtract the X and Y co-ordinates directly, you will preserve perfect accuracy. However since you use the 'Move at angle' action, which can take any angle at all, it calculates:
x offset = cos(angle) * move distance
y offset = sin(angle) * move distance
Trigonometric functions can only return approximate results, and work with radians, so there is opportunity for rounding error in both the degrees-to-radians conversion and the calculation of a trigonometric function.