Testnet4 has a fix for the difficulty adjustment that was causing blockstorms on Testnet3, but we kept the 20 minute exception: if the timestamp of a block is 20 minutes into the future, the block must have minimum difficulty. The idea was that it would allow protocol and app developers to mine themselves some testnet coins or to mine a non-standard transaction into a block to test what they are working on.

In the past couple years, people started trading Testnet3 coins on an altcoin exchange, which made it even harder to get testnet coins for free (but maybe more reliable if you were willing to pay). Testnet3 was already at a block height of 4 million and the subsidy had largely dried up, so between that, people trying to harvest coins from faucets to sell them, and various "layer2" projects testing on Testnet3, they made decent money.

This set the stage for the Testnet4 launch. Unfortunately, immediately after launch, someone started mining all of the Testnet4 blocks they could by exploiting the 20 minute rule programmatically: every time a new block with actual difficulty was found, they would immediately create 6 blocks pushed forward into the future by intervals of 20 minutes at minimum difficulty. Other people seem to also try that, so that there are a ton of reorgs now. Basically, a classic tragedy of the commons: someone tried to exploit the commons for personal profit and now it’s broken for everyone.