"An old sandwich"
Okay, I may hear something like that. Anyway, Polish is read exactly how it's written in 99,999% of cases. There is devoicing, true, but that's not something what we think about when pronouncing words, it's something that just happens without our knowledge. P and T aren't a pair, so such a pronounciation is not an option.
I'm afraid there's no other way than to accept that the TTS voice has quite a number of imperfections. Just observe the way the word is written, and remember that at least 98% of words have stress on the penultimate syllable, no matter how the voice pronounces them.
Right now the only thing we can change is disable a "write from hearing" exercise if the pronunciation is really confusing.