They are implemented as ligatures in Computer Modern fonts, so it is not unreasonable for them to not work with any other font that does not have such ligatures.
A couple of points: 1. It seems reasonable to expect Unicode-aware engines (luatex and xetex) to make this substitution, especially with the Ligatures=TeX option given to fontspec. (Isn't this the entire point of that option?) 2. Regardless of that debate, my example in the previous email shows the bug in action with the default Latin Modern fonts. Surely those should perform the substitution.
But beside this, it is not the point: the fontloader *has* an emulation and this emulation has clearly currently a bug.
But there is hope: Marcel just sent a pull request which hopefully resolves the problem:
Great news!