By default "Adaptive Difficulty" is on.

It's been on ever since update 1.50. You can read the exact patch notes here

https://steamdb.info/patchnotes/6146821/

and what exactly adaptive difficulty DOES is that it adds or removes the number of enemies in a room depending on how well you are doing. One of the devs themselves explains it as

> Hey there, the adaptive difficulty basically work on the number of enemies. If you're doing well, the game won't change and be just as you knew it before this was introduced. If you're hardly progressing, the game will start removing enemies until you make progress, and will add enemies until your reach the base difficulty if you're making progress with a reduced enemy count.