Goodhart’s Law really only applies to measures that aren't measuring the actual figure of merit but a proxy that is incorrectly assumed to be inextricably tied to it.
This is very common, though, and a real problem associated with it is people pick proxies without clearly documenting (or even thinking through) what they want to measure, why they pick a particular proxy, and how the proxy might fall. As a result, people remote from the decision (including the original decision maker) tend to incautiously apply the proxy because they aren't even aware that it is not the actual figure of merit.
The point of the law is that virtually every measure is a proxy to some extent. Finding a measure that exactly matches what you really want to optimize is almost impossible.
This is very common, though, and a real problem associated with it is people pick proxies without clearly documenting (or even thinking through) what they want to measure, why they pick a particular proxy, and how the proxy might fall. As a result, people remote from the decision (including the original decision maker) tend to incautiously apply the proxy because they aren't even aware that it is not the actual figure of merit.