' is considered to be an advantage, especially if it is even and pretty.
most coins that are extremely shiny have perhaps been a tad overcleaned, and many collectors prefer it when time gives them back a bit
of their character. this is not only for aesthetic reasons, but a nice toning
can highlight the features of the coin and add a certain depth, and there are those collectors will actually pay a premium for this effect.
there probably isn't much you can do to prevent this in the very long run, but you may find that you won't want to anyway.