IFR....
This has happened to me on the rare occasion as well, although usually after I have had the particular program open. I am only guessing, but I think that something to do with the icon gets corrupted and hence the change to some sort of default. All I do is delete the changed icon then create a new one as before without any further problems.
Probably just one of the many quirks with Microsoft that goes on behind the scenes that we all have to live with, with the result of one going mad trying to figure it out.
I hope this helps.
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The R. Sole