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.
Yes, and this is imho a problem of open type fonts. I do find it very difficult to handle sources which uses the real unicode chars, as there are barely distinguishable.
XeTeX and LuaTeX providing ways to emulate this with other fonts is already going out of their way,
I'm glad that the engines offer work-arounds around this missing feature in open type fonts. 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: https://github.com/u-fischer/luaotfload/pull/45 -- Mit freundlichen Grüßen Ulrike Fischer