And it was suspected to have issues much earlier.
Although it was about 40 years between the initial “discovery” of the Piltdown material (by someone seeking attention and later shown to be prone to faking archaeological discoveries) and the prominent publication of data showing that it was fake, there were scientific misgivings from the start, and by the time it was publicized as a fake it was already quite clearly not in accord with the actual evidence about human evolution. Carbon-14 dating is an important component of the disproof of Piltdown’s validity. The reality is that Piltdown Man is an example of scientific research making corrections.
For example, one early reason to be skeptical of the Piltdown finds was that one object was a mammoth bone carved into what looked like a cricket bat.