I've never heard of this before, Can you provide more information? Where did you hear of these "flash Bitcoins" that disappear in 90 days?

There are various altcoins or tokens that can be loosely referred to as fake Bitcoins. For example, BSV is often considered a fake Bitcoin. However, it's an altcoin with its separate network and market price, unrelated to the actual Bitcoin network. Also, there are so-called "Wrapped Bitcoins," like WBTC, which also aren't real Bitcoins but tokens on other networks that represent the value of real Bitcoins. In most cases, you can exchange them for real Bitcoins. Still, I've never heard of fake Bitcoins that vanish in 90 days.