Basically that is the problem where an attempt to measure or proxy something desirable incentivizes participants to optimize the proxy measure rather than the actual desired characteristic. There often is still some correlation between the proxy measure and the actual desired characteristic, but it is confounded by “teaching to the test” or “test preparation” that may result in higher proxy measures than the actual desired characteristic for some, and lower proxy measures for those who did not do such activity.
In other contexts, it may be called “gaming the system”. In professional sports, this sometimes results in changes to the rules to prevent exploitation of loopholes in the rules when such exploitation becomes too common, is seen as “unfair” by spectators, or makes the game less entertaining generally. Other places where “gaming the system” occurs includes politics (e.g. gerrymandering) and income taxation. College financial aid is another place where parents may choose to time financial transactions before, during, or after the tax years that college financial aid calculations look at, in order to get the most favorable financial aid calculation.