Hi Meneghino,
I configure the ChevronDisabledBackground of the Drop down control as RGBA(0,0,0,0), then when I set its Disable property to True, the chevron is showing as below.
It seems that it works as expected, I don’t do any other configuration on the Drop down control.

For a test, I set the ChevronDisabledBackground property as If(Dropdown3.Disabled=true,RGBA(120,13,140,150),RGBA(11,12,13,14)), then when I set its Disable property to True, the chevron is showing a color as below.

For another test, I set the ChevronBackground property as If(Dropdown4.Disabled=true,RGBA(10,12,13,18),RGBA(0,0,0,0)), without any configuration on the ChevronDisabledBackground property, when I set its Disable property to True, the chevron is showing as below.

Per my test, it seems that the chevron color depends on the color you have set on its ChevronDisabledBackground property.
Could you please check the formulas you have set on the Drop down?
Best regards,
Mabel Mao