Hi Enrico,

I fear I can't change mkgmap to fix this problem.
The problem is more or less bad luck.
In some cases, you have placed the points of the ways
more or less exactly between the points of the Garmin
grid, others are very near to the grid points.
If I got it right, we can only produce a straight line
in the garmin format when we allow to move certain
points by more than
2 metres, and in that case the WrongAngleFixer 
will prefer the correct position to the bad angle.

I''ve attached the grid as a gpx file, if you load that
into JOSM you probably see what I mean.

If you prefer "nicer" rendering on the Garmin map
you may move some points closer to the grid to solve
the problem.

Gerd