Windows is very jealous of its saved icons, it doesn't want them to change in any way. This is the problem I face every time I'm developing a program and I try to change its icon. The operating system associates the executable to the old one, and it shows that instead of the one actually contained in the binary.
Luckily, there is a simple command to refresh the icon cache without having to delete system folders or rebooting, as some answers found online suggest.
Other Windows versions