https://www.kitguru.net/gaming/matthew-wilson/turtle-rock-will-continue-to-address-back-4-blood-difficulty-and-balance/
 
In a community update on Reddit, Turtle Rock confirmed that it “spoke too soon” when it claimed that an earlier patch had solved the unusually high difficulty level. Currently, Back 4 Blood has an “unintended increased difficulty” level, and the team is “actively working on a solution”.
 
Previously, these difficulty spikes were attributed to a bug, which would cause Special Infected to duplicate and attack players in high numbers. It seems that trauma damage to the player is also being dealt too quickly. Speaking on difficulty and balance, Turtle Rock explained:
“Difficulty needs to be our ‘baby bear’ in Back 4 Blood’s Goldilocks story. It needs to be “just right.” We want it tense. We want it challenging. We want you barely squeaking into the saferoom with a horde hot on your heels and you feeling that rush. We do not want it unmanageable. We do not want it to be unfair. That is our mission.” Turtle Rock will continue tinkering with Back 4 Blood balance, adjusting the effectiveness of player weapons and cards, as well as the damage and spawn rates of infected. The studio has also unveiled its content roadmap recently, which will include free content updates, followed by paid campaign expansions in 2022.
 
At least Turtle Rock has acknowledge the issue and is working on it.