An interesting question to think about:
If a person ate matzah but it was tafeil to some other food so no bracha needed to be recited on it, is a person yotzei the mitzvah of achilas matzah?
A few months from now you can discuss the same question by sukkah: if a person ate bread on leil sukkos but it was tafeil to some other food so it did not require a bracha, is a person yotzei the mitzvah of eating a meal in sukkah?
What's the safeik? Saying a bracha is not m'keiv the mitzvah?
The issue is how to understand the din that ikar poteir es ha'tafeil. The Emek Bracha (and others) make the following chakira: is it that the tafeil is its own achila and should get its own bracha, but the halacha of ikar poteir es ha'tefil tells us that the bracha on the ikar suffices, or does the din of ikar poteir tafeil mean that the achila of the tafeil is not an achila -- it's like you just ate the ikar, and that's why only the ikar gets a bracha.
There are ra'ayos both ways, among them the simple diyuk in the language of ikar POTEIR es ha'tafeil, which implies there is a chiyuv bracha on the tafeil that is being fulfilled, not that there is no chiyuv.
In any case, the sefer where I saw this question raised wanted to say that the issue by matzah or sukkah hinges on this chakirah. If the achila of the tafeil is not an achila, then you haven't been yotzei matzah or sukkah; if it is an achila, just the bracha on the ikar suffices, then you would be yotzei.
I leave it to you to mull over whether the chakira fits and what the proofs are one way or the other.