Duotone on some icons doesn't take second color

[RickStrahl]

Some icons appear to not be configured for the secondary color when using Duotone mode.

Here is one example that doesn't respond to the second color (this renders in Firebrick but doesn't fill the square in white:

<fa6:ImageAwesome Icon="Duotone_CircleStop"
           Height="18"
           Margin="0 1 6 0"                                                                                                                                          
           SecondaryColor="Firebrick"
           PrimaryColor="White" 
           />

All of the icons you see are using Duotone with two colors and it works, except for the Stop icon. The icon does have a duotone mode in FontAwesome.

I've run into this with a few others as well, but I can't recall which ones at this point. It's fairly rare.

Other

I'm using FontAwesome6 Pro icons in font mode.