How can we reproduce this bug?
Use any dark background ($callout-background: #000) for callout, with $callout-background-fade: 0.
What did you expect to happen?
Callout font color is $callout-font-color-alt.
What happened instead?
Callout font color is $callout-font-color.
Having looked at the codebase it looks like you are right and this was never fully implemented. This will need reworking a bit by the look of it.
I believe this was resolved in 6.3, closing.
Most helpful comment
Having looked at the codebase it looks like you are right and this was never fully implemented. This will need reworking a bit by the look of it.