If you replace a voice (any voice) in a style, then reselect back to the original voice, it may no longer sound the same.
That's because the voice may have had customised voice settings applied to it within the style file, which are lost when you change it.
Depending on your keyboard model, the drum kit may even have had individual drum sounds changed or replaced.
The only way to get it back, without disturbing the rest of the style which you have have customized, is to use the style assembly function to graft the part back in again from a copy of the original style.