The problem is :
The caret is moving from left or right to left (or left to right) when I use the spin buttons.
the spinmode is set on advanced, so when the caret moves on digits, the spinbuttonstep is changing !!!
At first the spinbuttonstep is 0.5 because the caret is close to the decimal digit… but after à few clics, the spinbuttonstep is now of 5 instead of 0.5.