You're skewing the results by a bad test.

Don't "ask your partner" to say a particular word or phrase. The very act of asking that will have changed results. (This is experiment design 101 stuff here!) Ask her to read a lot of stuff that has "hot potato" in it in various places. (We tend to use sandhi in flowing streams of speech, not isolated clips.) Or, ideally, engage her in conversation and get her to say "hot potato" naturally as an organic outgrowth of the conversation.

But ... make sure you record what she says. Your own brain, as a listener, fills in stuff that's not there while removing stuff that is. You have to play it back, concentrating on only the sounds, not the words, and do it repeatedly, ideally isolating this one phoneme at a time.

Really, sandhi is a thing, and it's a thing that literally every native speaker of every language in the world uses. There is variance by dialect, naturally (entire phones vanish or come out of nothing from dialect to dialect), but some elements of sandhi (like consonant assimilation) happen no matter what your dialect unless you're specifically concentrating on having it not happen.