both forms for party that you mentioned are permitted according to Czech language institute in written language. I have never seen piano with á though and according to the Institute only piano is possible. I can imagine however that many people write it with á as these borrowings do not sound czech and many people can be confused because usually we read the words the way they are written. Then people reflect it incorrectly in the written language as well.
Thank you, I think maybe party and piano are better - as these are not really Czech words. As most young Czechs speak English, the consciousness of the English pronunciations (although piano is ultimately Italian) will be a factor.